The latest is healers who pretend to try, but don't really so you lose quickly.

Had a WHM earlier stand there casting holy instead of healing. He said that he was trying to stun them off of him; right.

Had another WHM stand there and cast Medica (not even medica 2) when only a BLM was being focused, rest of the party was full HP. He then proceeded to cast Cure - no, not Cure 2, but Cure, when the BLM was at 40% HP and being heavily focused.

Things like this could easily be dismissed as "oh sorry, i'm new", or "I'm just bad, sorry". Get real though, it's no different than throwing a match by not healing.