It happens. My old Dinotte 140R with the AA packs - the AA packs would rust out in the rain. I replaced them 3 or 4 times, Dinotte's response was "dry them out ASAP if they get wet, or they will rust"

I had a Serfas USL-TL60, which was about $55, and it had water inside it after just one ride in the rain, and yes, the USB cap was on. Serfas replaced it when I sent them a photo.

The waterproofing on the SuperFlash lights is notoriously bad, a lot of people have trouble with them getting wet inside. I have little experience since all of my SuperFlashes are just sitting in a box since they're really not very good by modern standards (their only real good feature being that they run a heck of a long time on AAA cells - but they're not really very bright).