If you accidentally hit one of the automation lines on a Group or AUX or Instrument track in the project window (volume, pan, etc.) that’s NOT automation-enabled and has no automation on it yet, then it puts a node at that point and snaps to a new value, which is normal behavior. However, the problem in this particular situation is that you canNOT undo that and have no idea where the original value was – you can’t go over and undo it in the mixer either…your original value is gone. Hitting those faint grey lines can happen easily by accident in complex projects, and it should be able to be undone completely when it does.
Click anywhere on a Group or Aux or Instrument volume/pan/send/etc. automation line in the project window that does NOT have any automation and is not automation enabled, and drag to a new value. You will see that when you undo, the node goes away but the line stays at the new value, and there’s no way to recall where it was. This is a very major workflow killer when it happens, for very obvious reasons…and it’s VERY easy to do in even semi-complicated sessions.