During a "C" check on a 757 a few years ago, a stringer near L4 was found to be fitted undrilled (ergo, not rivetted). The aircraft was nearly 20 years old at the time and had had several heavy checks in that time. These things happen.
I also recall a Boeing engineer telling me that he had found an elevator pulley in an in-service 747-400 that was retained by "Clico" clamps instead of Hi-Loks. (Clicos are temporary clamps used during metal work)