I don't normally tip the housekeeping staff, as a recommendation from my friend I left a tip on the bed the first day. This prompted one staff member to remind us everyday to leave a tip. This was annoying.
The same staff member charged me extra to clean a dirty towel. I had been walking in dirt the previous day and used the towel to wipe my shoes. This was dirt, not grease or any other substance. The amount wasn't much, but this was going into her pocket, not the hotel. My friend and I informed the hotel when we left, so I don't believe this will happen again.