Bills can be mistakenly added to a dryer mid cycle.
If you move a pile of bills too close to an active dryer, the bill pile can clip through the sides of the dryer and be added to the in progress pile within the dryer. Essentially, (and as is the case with what just happened to me) if you keep a pile of dirty bills too close to your dryer, they can get added to the already cleaned bills that are in the middle of being dried, causing you to have to sort back through the pile and separate all the freshly cleaned bills with the dirty ones that shouldn't have been added to that pile.