In Cochabamba beware of a fake...
In Cochabamba beware of a fake policeman searching you for drugs and false dollar bills. He leaves the small bills in the wallet and takes away the big ones.
In Santa Cruz take care of unknown people who share drinks with you, mixing them with tranquilizers and rob your money. In the outskirts armed robbery also takes place quite often, but not in the center.
Carry you passport at all...
Carry you passport at all time, Bolivian policemen sometimes harass tourists so they can get some bribe. Make sure to keep your documents in a very save place at all times and make copies of it before going to Bolivia.