I haven't exactly misused my tools intentionally, so I can't say for sure, but it's a common problem using the old MEGAs, the ISP pins are also the SPI peripheral, so you usually need some facility to isolate those pins from attached peripherals (if used) when programming. Or only program out-of-circuit.
If so, it's extremely likely that you 1. neglected to trace the circuit and see what else is connected to these pins, and 2. the connected device asserted MISO during programming, corrupting commands/data. I don't know what commands this would assert, but it might well be an erase cycle is among them. It could also be that it got programmed with trash, or that the fuses are all wrong.
Lesson: if AVRdude reads all zeroes, you have data corruption. You will not read or write anything meaningful by forcing to proceed. Check your connections and try again.
Tim