As a DM, I love to play with the rules quite a bit, especially with magic, and my players always enjoy a little improvisation. One player especially, a Wood Elf Wizard, is constantly drunk and/or high. Well, as you can imagine, casting while under the influence can lead to some pretty wacky stuff. Case in point, the team was taking on a final boss last session, and had gotten the boss down to 1 hp with one lacky (a weaker goblin) left to deal with. Imagine my surprise when the aforementioned player turned to me when it came to her turn and asked, "Can I cast Jolly Rancher on the goblin near the pit?" Of course, the answer was absolutely yeah! With her drunken (improvised) casting I have a table for her result, and the only way she actually achieves a desirable effect is to roll a nat 20, which is exactly what she rolled. I described how the goblin tried to catch the Jolly Ranchers in his mouth, succeeded, and then choked to death (he had only 1 hp left anyway). Luckily, that totally unfair roll was balanced by another drunk cast in which she tried to dye her hair but ended up cutting half of it off!
I don't know how to get the table onto the forum (I may be young, but I have the technological knowledge of a 60 year-old!), but I can improvise.
1-5 You fail in achieving the desired effect and potentially do great harm to yourself and/or others. 6-10 You fail in achieving the desired effect. Instead, the opposite, or at least a vastly different, effect occurs. 11-15 You succeed in achieving the desired effect but that effect turns out to do more harm than good. 16-19 You achieve the desired effect. Make a DC 17 Con. Saving Throw, & fall unconscious for 1d4 hours on a fail. 20 You totally succeed!
As an example for context, our wizard attempts to open an iron door by turning it into goo and letting it slide away.
1-5: She accidentally transforms the floor underneath her, putting her and her nearby friend in a quicksand-esque trap. 6-10: The door melts on the edges and then cools, welded to the frame. 11-15: The door is turned into goop. Acidic goop that spills onto the party. The door's not there though! 16-19 The door is successfully gooped but she falls asleep and wakes up with a massive hangover after an hour. 20 The door is gooped and splashes onto the floor harmlessly waiting for the party to walk through.