Stuck Coin in the Telephone Scam
Often times, people will gather change by finding old public telephones that do not have a guard in the coin return slot. After finding one of these telephones, they stuff the coin return slot with paper to stop the coin from falling out when someone tries to get their coin back.
Also, there have been unconfirmed rumors of IV drug users placing used and infected needle tips into these coin slots so that when someone puts their hand in to check for money, they get pricked with the dirty needle.
How to avoid this
Whether or not this is true, you may want to push the change flap open with something other than your fleshy finger to avoid any potential danger.