Many people will have seen this variant on Mellowmellon's blog. This was not the first place I encountered these puzzles though. Back in 2006 they appeared on the Dutch Puzzle Championships. We had a short round with 4 of these puzzles. I really liked the idea back then, but hadn't really come back much after that. I found it interesting to see the idea had popped back up afterwards.
Rules for Slitherlink
The clues work as a normal Slitherlink puzzle, except in every row and column exactly one clue is incorrect.