It is customary to suggest that, every now and then, life comes and bites you on the arse.
To me, this seems an unreasonable stance: I do not model life as an active agent with malevolent intent. Life is a passive force that simply sits here, getting in your way, while you try to get on with things. I would rather use a different metaphor: every now and then you bump your head against the barrier between your intentions and reality.