I love critical hits. Back in 3.5 I used Paizo’s Critical Hit deck to add more elements of fun to these hits, so it wasn’t just extra damage.
I never did get to try their Critical Fumble deck, but I’m intrigued by this opposite of Critical Hits. I made my own chart that I’ve used with much success in my home game and cons. People actually look forward to failing. The version seen here is half the list that I use at home (its normally a hundred option long) that I’ve cleaned up for use by others. I think it represents the best of the list.