Most recently, it was with my son's sippy cup. Henry never really took to the bottle, so he has been drinking from a cup from some time. But every time I went to clean one out, I got so grossed out by the slimy black mold that aggregated in the nozzles of the thing. And there didn't seem to be any way to get it out.
I tried soaking them, boiling them, trying to angle a sponge up in there, using an old toothbrush, to no avail. And then one day --eureka! I popped a Q-tip up through the nozzle, swiped it around, and all that ooze just came right off.
What stupid solutions to annoying problems eluded you for too long?