Examples:

Example 1:
Gonna cast a debuff on an opponent.
Accidentally have my team mate selected. -> Get can't cast on team mate message.

Example 2:
Gonna cast a debuff on an opponent.
Accidentally have myself selected. -> Whammo! I'm debuffed!

Example 3:
Want to shoot monster!
Accidentally have self targeted. -> You can't shoot yourself!

Example 4:
Want to jump into room quickly cast a debuff on monster.
Jump into room, hit tab, tab doesn't select monster who is in my face, but instead leaves target on me. -> Whammo I debuff myself.

Argh!