Custom ROM gang: puts off updates for three months because you can't be bothered to actually download the package and re-flash it manually, and it might also screw up your user data and require a reinstall
Granted, this used to be a way bigger problem in the Kitkat/Lollipop days, when CyanogenMod hadn't become Lineage yet. Nowadays OTA updates work pretty well, but occasionally it'll still tell you to flash it manually.