The ramifications can be dire or laughable depending on your airline/seniority/record, etc. Senior Captain from back home got no more than a slap on the wrist when it cropped up that he'd doctored his log 20+ years ago in his pre-airline days. He's a good bloke, had a great record and was due to retire in the next few years so they let it slide but the fact of the matter is that his career progression wouldn't have happened at the pace it did had he been honest.
For the rest of us...keep your nose clean and don't be cheeky, it all catches up.