Why Wouldn't It Be Clean?

I went through the Tim Hortons drive-thru on my way to work this morning.  Being the one who likes hot coffee (not necessarily being earth conscious), I bring my own travel mug -- ironically purchased at Starbucks.

I handed Tim (seriously, that's his name) my mug and upon opening the lid he quipped to me "Hey!  You're in the Clean Cup Club!"  I asked him what he meant and he clarified that I had a clean cup.  My slightly incredulous response was "Why wouldn't it be clean?"  Tim said that I'd be surprised at how nasty some of the travel mugs are.

There have been:
-sludge (of varying heights)
-rank milk
-leftover coffee
-mould
-crawly things
-science experiments

Gross.

I can't even drink out of the same water cup twice.

Nasty.