Loved these bottles. Bought two packs back in March. I just noticed some mildew on the rubber ring inside the lid. Odd bc I wash and dry immediately. I checked them all and I can see some mildew spots under several of the other rubber lids. How this happened as I use them to store water I’m not sure. I just looked again while writing this and I can remove the rubber seal using a toothpick or fork to clean them. I don’t buy items that are hard to clean, but this will take some extra time to clean and will be tedious because the rubber seal is small. If you don’t keep up with these it would be easy to lose them and possibly prevent you from being able to use these without them leaking if the bottle wasn’t upright.