
Here's a wild one... the soft sleeve/holder for a glass water bottle...
I've had one of these water bottles for about 15 years now: https://libmagazine.com/wp-content/uploads/2014/01/699.jpg
It's an absolutely incredible water bottle, and I wash it out very, very regularly.
However, every time I wash it, I take it out of its sleeve - of course, right?
...and I never washed the sleeve...
Today I got the wise idea to do so. First I let it soak in a sink full of soapy water... when I came back, the water was dark brown and very murky. So I drained it, rinsed, and did it again. Same result...
I just spent about 15 minutes scrubbing it under hot water with lots and lots of soap, and dirt kept coming out of it the whole time... it's still not totally clean, but my hands hurt, so I had to stop :D
If your water bottle has a sleeve, holy shit please wash it regularly! I will be doing so from now on!!!