double spending problem
Double spending commonly refers to the situation where you send 1 BTC to Alice but you privately create a fork and on that fork you send the 1 BTC to bob. If your privatefork mines enough blocks to surpass the main fork (i.e you have the majority of hashpower or you are really lucky) you publish it and all the nodes consider it valid since it is the longer chain. As a result Alice loses her 1 BTC.