My 20mo has started freaking out when certain things don't follow his expectations while playing. E.g., if the electric train derails due to a faulty track, or if he spills something by mistake. He's usually pretty easygoing other than this. He seemed especially tired this evening, and he was flipping out so much that I had to cut off two activities in succession and finally just call an early bedtime.

Is the likely to be a personality trait, or is this the start of the terrible twos?