The following examples are all selectors which will successfully build but obviously fail regardless of the surroundings, or which don't make logical sense. Usually the parser should catch these errors beforehand:

• @e[type=pig,type=cow]
• @a[gamemode=survival,gamemode=creative]
• @a[sort=arbitrary,sort=random] (latter sort overrides former here)
• @a[type=llama] (type should be considered invalid for player selectors, as limit is for @s)