A word or phrase for an unremarkable event that occurs with uncanny frequency

If I understand correctly, you're looking for a phrase that describes a surprisingly frequent, but mundane event.

The underlying assumption of this answer is that it is perceived as frequent. Thus, under the umbrella of cognitive bias, it would be called an illusory correlation event.

more on cognitive bias

Agreeing with andhrimnir, the fact that we imagine an event, like a random shopper dropping a grocery bag, as more frequent than it really is would be called the frequency illusion:

n. The tendency to notice instances of a particular phenomenon once one starts to look for it, and to therefore believe erroneously that the phenomenon occurs frequently.

It is related to the "purchase illusion": when you buy a new car (or dress) you suddenly start noticing that everyone else already has one.

Seeing Men in Black on the tube in a hotel room would not be frequency illusion, because it is on one of those free channels that plays old popular movies over and over again.

Back in the Navy, we had a phrase: "The 50/50/90 rule". It had a more negative connotation but it meant that given a 50/50 chance of two possible outcomes, the worst would occur 90% of the time. I'm sure that our belief in this was a case of "confirmation bias", but we had a name for what wiser people told us was not happening.