It will cause a critical error during boot if the device isn’t given the nofail mount option, which is not included in the defaults option, and then fails to mount. For more details, look in the fstab(5) man page, and for even more detail, the mount(8) man page.

Found that out for myself when not having my external harddrive enclosure turned on with a formatted drive in it caused the pc to boot into recovery mode (it was not the primary drive). I had just copy-pasted the options from my root partition, thinking I could take the shortcut instead of reading documentation.

There’s probably other ways that a borked fstab can cause a fail to boot, but that’s just the one I know of from experience.