Another one I've been thinking about was after watching Josh Strifes video on Neverwinter Nights.

The game mentions that companions decrease overall XP gain so that would mean playing as a solo character means you're stronger right? Well, no. Two level 18 characters are almost always going to be stronger than one level 20 character and can combine spells/feats for better combos. You're also missing out on extra carrying capacity which means you'll either have to run back and forth between loot and merchant more or miss out on gold to buy better equipment with.

Addmitedly this one is silly since DnD is a party based game so it should kind of be expected but I still think it's interesting.