|*Kandinsky, Yellow, Red, Blue|
The idea that a moped is safer than a motorcycle is insulting, because the only difference is that one goes faster: Therefore, you are saying the rider is too irresponsible to drive a safe speed on the motorcycle, whereas the moped just limits his or her ability to get out of the way.
There. An argument. Now, Joesky's Rule.
You probably know that synesthasia is the state of experiencing one sense or perception as another. Tasting colors, seeing sounds, etc. What you may not know is the condition has many interesting variations. Several of them could be very entertaining after-effects of miscast spells, potions, fungal spores etc.
Here's three I thought would be fun as temporary effects:
1: Grapheme → color synesthesia
The PC now associates all letters, numbers, and sometimes words with specific colors. For instance, a "P" always appears red, while an "R" appears blue, etc. This is true no matter what shape the letter is in, or what color it really is: If the PC can tell its an R, then it is blue. This is extremely disorienting and makes it impossible to correctly cast a spell from a scroll, or memorize a spell from a spellbook. However, it makes it easier to spot text that might otherwise be overlooked - while everyone else sees a dusty rock, the PC sees a dusty rock with a big rainbow "BEWARE" on it.
2: Sound → color synesthesia
Sounds produce colorful shapes and patterns on a "screen" in front of the PC, obscuring vision. Sharp or loud sounds are bright, while deep or soft sounds are darker. The size of the shape corresponds to volume - very loud noises essentially blind the PC. All shapes fade immediately when the sound making them stops. Music forms complex patterns of movement. In a situation with lots of noise, like a carnival or a battle, it's almost impossible to concentrate (raise the difficulty on all rolls where this would matter). The upside? It's easier to notice any sound, like something or someone trying to sneak.
3: Lexical → gustatory synesthesia
Speaking, hearing or reading certain words evoke tastes in the mouth that are specifically associated with that word. For instance, saying or hearing "dagger" might make a PC taste blood, if you go the route of having past experience influence the result, or eggs and bacon if you want to be more realistic. To make this one actually interesting in-game, I recommend picking a random D&D book, then a d4 and two d10s. The d4 is your hundred place, the first d10 is tens, and the second is ones (so, you should have something like 2 8 1 or 4 2 7). Flip to that page. Roll a d20, and go to that paragraph. Roll again, and find that word within the graph.
If anyone says that word, the PC tastes spoiled milk and must make a Fortitude save or vomit.
Roll to find another word on that page, or another if you want. That word tastes like the first time the PC ever tasted "sweet" in their life. Any spell or effect that relies on or creates negative emotions is negated.
Roll up three more words. These taste like ham, blood and grass. No effect... these are just to tip off the player that something is up.
Also, if anyone trying to control the PC's mind uses one of these words in their command, and the command is in the PC's language, the spell is broken. They still vomit, though.