I'm a relatively newbie diver (only 80 dives), but there have been a few times where something unexpected or new happens that can send you into at least a mild panic. Recognizing this panic and how to recover is the critical thing.

A most recent example was on my last trip I tried UV-light diving, which is when you only use UV-light with a filter on your mask to protect your eyes. You get to see some incredible new things. The divemaster stressed how important the filter was to protect your eyes. During the dive the filter on my mask came off. As instructed, I immediately closed my eyes and Waited for the divemaster to notice and tap me to let me know they had switched back to white light and it was safe to reopen my eyes. That half minute felt like an eternity.

A number of unusual situations can trigger a panic. Primary regulator failure, your mask being knocked off, unexpected fauna behavior, becoming lost, losing your group or your buddy, unexpected currents. All of this is covered in the training/certification, but not all training is equal, adequate, or regularly revisited.