Playing an phony (e.g., deviaters*). Bluffing might be done purposefully in order to test your opponent's word knowledge or unintentionally because of your belief that a phony word is acceptable. Bluffing is perfectly acceptable in organized Scrabble. See Phony.
Tricking other players into thinking that you have a better hand than you do
The act of deliberately playing a phoney word. This is completely ethical and is a weapon used by many experts, even against other experts.