From a travel forum that I use:
In Paris, they have what look like large green porta-potties on the major streets. You have to pay to open the door, so sometimes if you're short on coinage, it can be tempting to have another member of your party just slide in after you're done, thus eliminating the need to pay again to get the door back open.
Don't do it.
When the door closes after the toilet has been used, the door is automatically locked and the entire cabin is sprayed down with disinfectant, then flash dryed. It makes for very sanitary facilities, but it also creates a very sticky situation for the "thrifty" second person.