One of my daughters got a towel bag as a present. That's what I call it because I have no idea what it is actually called. I thought it was a good idea at first...
It's a beach towel that folds itself into a bag with a strap. That way you can actually stuff your bathing suit and goggles (or whatever) into the bag and throw it over your shoulder. Cool. No extra bag to hold everything needed.
Until my daughter stashed it into her room for days...used it another time...stashed it in her room again...repeat, repeat, repeat. I found it one day, pulled the towel out, and was disgusted by the huge black mold spots. OMG! I asked her why she was continuing to use the towel and she said she thought it was dirt!!!! EW!!!! I tried bleaching the thing, but no luck, so it's gone on to a better place now.
This is just one of those ideas the inventor didn't think through. Many kids just don't think about things like mildew and mold. And I never realized it was missing out of the wash.