New patch new bug: Mercy will sometimes have her options for Guardian Angel flipped between rounds (never the first round) despite appearing the same in the menu. This can be... pretty bad as instead of flying to a teammate you'll just dive directly into the ground.
If this happens, you have to go into the character-specific controls and change the option from what it's supposed to be and then back to what it's supposed to be. To anyone playing as Mercy, I would recommend testing that Guardian Angel is working like you expect it to in the warmup.
From looking briefly at the Battle.net forums it seems this can also affect other options, like Ana/Widow scope toggle, but that seems less serious.