The A-320 family is a great aircraft to fly. The automation does so much for you.
But it can surprise you with anomalies, especially in the first year you fly it when you forget to activate the approach, fail to reach planned cruise altitude and don't re-cruise, don't know the implications of alt star and vertical speed, et al.
If you're like me, you learn by being surprised, then learn to avoid the surprises. For example, I know that when the magenta diamond is below the horizon line, it's bad.
But my main point is this: WHENEVER YOU ARE SURPRISED THE MOST IMPORTANT MEMORY ITEM IS:
AUTOPILOT OFF
AUTO THRUST OFF
FLIGHT DIRECTORS OFF
IF AIRSPEED IS UNKNOWN, SET 90% N1
Then figure out what is going on and fix it.