Use this phrase to describe a negative characteristic that a person sometimes has:

She can be really hard to work with sometimes.

He can be painfully shy when he's around people he doesn't know, but to those of us who know him he's actually really talkative.

Saying that someone "can be ___" is a good way to say something negative about a person, while still appearing to speak positively about them. So you might use this carefully to complain about your boss with a coworker, for example.

It's common to use "sometimes", "at times", or "from time to time" along with this phrase.