Just opened a new jar of jam, only to find mold in it.

With that untouched surface, you might be able to get a replacement if you still have the receipt. Depends on the store and if you're polite about it. (You could probably do it by being rude, but I'd hate you.)

You could probably get a replacement even if the jar is completely empty. I used to work customer service in a grocery store. One time a guy returned an empty wine bottle. I gave him a full refund. Why? Because the company steals from their employees every single day. I'm just evening the playing field a bit. Just don't be an asshole, and you'll probably get what you want.
- Yup yup yup. Former service employee. You yell in my face and complain to my boss, I'll give you the bare minimum of what you'll consider as "what you want" for you to shut the hell up. Come up and level with me, treat me like a person that you are asking for help, I'm gonna do literally everything I can to get you what you're asking for and more.
I went to Tesco (a large (perhaps too large ((brackets)) grocery retailer in the UK) a while back with some raspberries that hadn't even lasted until I got home before going mouldy.
Literally, all I wanted to do was show the customer service desk lass the issue, show them my receipt, get a replacement, and fuck off to make the interaction as easy as possible for them.
An honest appraisal of what the problem was; some witty banter; and a general understanding of the "don't be a dickhead" principle to frontline staff meant that she kept hold of me for ten minutes, noted down details and times and whatnot, and it ended up with a £10 apology gift card in the post - which in turn purchased a rather nice bottle of wine.
Where I'm going with this: I cannot fathom how people cannot be polite about issues like this. I've never understood how being a wanker to someone whose job it is to sort issues out somehow nets you a better end result. Perhaps the best advice I'm going to give my kids is to spend six months or a year of their teenage starts to their careers in an inbound call centre in a field they're interested in - yes the job can grind you down and yes the pay sucks, but the basic life skills and soft people skills it teaches you is fucking invaluable.
tl;dr: solid advice, ask nicely, don't be a twat, and you'll probably end up with a net result better than you started.
- “Ask nicely and don’t be a twat” sounds like an awesome t-shirt.
Just adding that I'm a current large supermarket employee, and customers who treat us like human beings are so incredibly rare that, when I come across one, I will go up against the gods themselves to get you what you want.
- And this is why i do my best to be the least imposing on workers at stores. I even will fix a stray cart
You'll probably get a lot more by contacting the manufacturer. It's not uncommon for them to give a bunch of free stuff.
Plus, it alerts them to a potential production/QA issue so they can address it.
- Sent them an email.
Yeah 100%

Call the manufacturer -- if it's commercially produced, there's usually a number on the jar somewhere. I've done that when I bought cereal that had a badly sealed bag inside and got sent coupons worth more than the cost of the item for taking the trouble to report it.

Normally I wouldn't bother on something that is pretty cheap, but if one failed to seal, they really need to sample the batch and see if a recall is necessary. Botulism is no joke.
- It was properly sealed, the little button was down, and it made a pop sound when I opened it.

I got one of those once, I contacted the grocery and manufacturer with the receipt. Grocery store promptly replaced it, while the manufacturer sent me a box full of random goodies and an apology letter

My grandfather complained to a company about broken pretzels in his pretzel bag and they sent him a case of pretzels and not one was broken.
Some stores will even double your money back.

I was told by a food research lab you could scoop off the top layer (oxygenated) and consume the rest of jams and sauces.

I'm not sure I believe them. They were growing large quantities of insects in a smelly, dank room for protein.

This is true, but I wouldn't do it when the jam is very liquid. And if your jam starts to mold fast, it means it doesn't contain extra chemicals to keep it good as well as much less sugar. So it's better if it molds faster. Just keep it in the fridge and eat it fast. Many store bought factory jams can stay good for over a year after being opened. Imagine what it takes to keep it from molding.
- No chemicals are necessary, home made jam can last for ages too. More sugar is better, for the jam, as a preservative. Not for oneself but like just eat less jam
I've done this with jam without issues for may years, my parents taught me. It's fine.
No. Look I don't care how many other commenters are agreeing with you, it's wrong. The tendrils go deep and just because you can't see them doesn't mean they're not there and not a toxic surprise waiting for you.
Just snort the mold off and eat it like a champion

moldly infuriating

That's what you get with berries concentrate (a supply-chain thing) and sugar.

But not so with self-made jam. Mold hates acidic environments. The jam rather dries out than getting moldy.

Not true. My mom's jams get moldy all the time. Maybe not all jams are acidic enough.
My moms jams do get moldy though.
- What jam is it? My mom is a farmer and makes all kinds of jam, never saw mold there the last 30 years.

Unless it's really old, just return it.

I just got it yesterday, so I'll try.

Extra protein.

Extra protein.
That’s disgusting.
You should always scrape the visible part off, and keep going until there are no more visible spots. Try not to skim too much of course.
You’ll still get all the extra protein from the mycelia but without the green fruiting bodies. 🤮

any sign that was opened before?

well there’s the mold
- Yeah, that’s not very typical. I’d like to make that point.
Might have had one of those pop up buttons to show the seal is broken. Ideally a stocker would see it, or the customer when selecting it, but I wonder how many people look that closely.
- I do, also every person I know. It always pop loudly when you open it.
- The pop up button was down when I opened it.
No. The little button thing was down.
- sad

If its in date you should probably report it to health and safety.

It was bought yesterday, and the jar was fully sealed.
- ya I'd definitely report it to your local health authority. I'm not sure how it works at all but as I mentioned in another comment they can ID what it is and follow up (eg is this a one-off or is there a whole batch that is contaminated?).
Do you happen to have the forms required?
- no but they can ID what it is, investigate the root cause and monitor this sort of thing.

Did you test the button on the lid before opening? I'd wager it was not properly sealed.

It was properly sealed. The button was down, and it made the satisfying pop when I opened it.

If unopened before this is a sterilisation issue. If opened before, it may be because of dew water from chilling

Yesterday's jam!

Comments saying contact the manufacturer, the state health services...
Or chill, remove the mold, smell, taste, if everything seems good eat it. Inform the store.

I've googled it just to be sure, and it's safe.

*not a doctor

Actually this can be risky. Mold might be cut from hard foods because it usually doesn't penetrate too deep, if not too overgrown. Soft foods tend to have "tendrils" all over that are often not visible to the naked eye.
- Though how risky is it in reality? Eg for bread, if visible mold means there's also invisible mold, wouldn't that imply that there's a period with just invisible mold that goes unnoticed and eaten? We're constantly inhaling and consuming mold spores anyways, so is this more of a "I know it's there and thus deem it icky" or "if it's soft and has mold, toss it all, and hope that you don't get unlucky and eat mold you couldn't see in the first place"?
I always assumed that the toxicity is in the spores, so I second this...

What brand?

Old food do be like that

Probably was sealed in a hot water bath instead of using a pressure cooker.