Expectation of healing. On many occasions, tanks get away with aggressive plays due to supports healing them and this may cause them to get into the bad habit of going in deep whilst expecting healing.

Oh God, so much this, I'm a healer main (any healer, best results with Moira and Ana) and the number of times I tell my tank to come back or wait for us and they're like "nah it's ok, I can handle it if you keep on healing" and yeah, sure, here we go, you survived, I survived but everyone else is dead and I used all my resources on your ass (orb+spray, nade, boost, armor) and now we're 2v6. What now?
That's generally my cue to switch to Lucio or Brig if I was playing Moira or Ana to force them to stay with us. (I only play Mercy or Zen when someone already picked Ana or Moira and then overextending tanks are not my problem, they'll die whatever I do so I leave that to the other healer and focus on boosting damages and healing squishies)