My next door neighbours are lovely. They have a DS the same age as DD, and while we aren't proper friends, we get along nicely and occasionally have a few drinks in the garden with the DCs. All very polite.
They (and we) use those munchkin non spill toddler cups... And a few months ago, their DS left it in our garden, so I washed it before taking it back round.
When I took the top apart to put it in the dishwasher, it was completely full of black mould! I scrubbed it, soaked it in Milton and even dug our old steam steriliser out to get it nice before it went home.
They clearly don't separate the non spill part from the lid before washing it, but I'm far too polite to tell them. I've dropped loads of rubbish hints along the lines of "oh those cups are tricky to clean aren't they?!" and even made up a reason to wash DDs cup at their house so they would see about pulling it apart.
But the same thing happened again last week, so I've now "accidentally" brought 2 home with DD this week for a proper clean. I even said when I took one back "I've taken it all to bits for a proper clean under the seal" and the mum just nodded and said thanks, like it's a standard thing to do. But it clearly isn't as the cups are all vile.
DP found out today and called me the dettol ninja 
But he is also too stupidly polite to mention the mould.
How do you direct folks confront issues like this without offending anyone? I'm honestly useless.