Many years ago an experienced pilot hired a C172 from a long since defunct flying school at Essendon. Maintenance release showed nil defects. En route to Point Cook via Westgate Bridge, ATC advised the pilot to switch on the transponder. The pilot checked and confirmed it was already on and had been since take off. ATC got upset and said it wasn't operating and that it was third time that week this particular aircraft had a u/s transponder and it better get fixed otherwise they would deny airspace.
The pilot decided to return immediately to EN and wrote up the defect and asked the CFI/owner why had he knowingly despatched this aircraft several times with a dodgy transponder? His reply was it was going to get looked at at the next 100 hourly and that so far ATC had always let it go. That same afternoon the CFI sent a student pilot off on his first solo cross-country with the same dodgy transponder. He said it saved him money by waiting for the 100 hourly.
What's the bet similar things still go on in GA and in remote areas?