i thought it would be more common than what you guys are writing so far. for example...

putting your cell on the toilet paper dispenser
putting bags/purse on the counter in the washroom
-- also opening the washroom door and then grabbing your bag
bus floor when it's busy
work/office floor (i do that often)
school floor beside your chair
when your trying on clothing at the mall
friends or your car floor
doctors office

etc..

note: even if you don't put your bags on the floor i still think they can get really dirty... for example: if you are riding the bus/skytrain and you are holding the bar, then your strap falls down and you move it with your hand to fix it...