I haven't accidentally deleted a bunch of data yet (which, considering 99% of my interaction with Linux is when I'm SSH'd into a user's server, I am very paranoid about not doing), but I have run fsck on a volume without mounting the read/write flashcache with dirty blocks on it first.
I did, and it was fast. I was a complete noob, so I thought rm -rf /* would delete everything in the current folder. I hit Ctrl + C, but it was too late. Took a few seconds to wipe out the whole system.