Use the phrase "___ has been known to ___" to describe something that sometimes happens or has happened a few times in the past:
They 've been known to rehearse 10 hours at a single stretch.
I'm not sure if I trust you. You have been known to make promises and not follow through on them, you know.
The phrase "__ has been know to ___" can either sound very careful, or sound funny and playful. So you could use it to carefully talk about mistakes your boss sometimes makes:
She's been known to miss important client meetings from time to time.
Or you could use it to joke about something embarrassing that you do a lot:
Yeah, I've been known to buy a new pair of shoes every now and then.
The joke here would be that you actually buy new shoes all the time, and it's funny that you're trying to sound like you don't do it a lot.