So far, I could reprogram a circuit with an ATMEGA48PA without problem. Until I got (after apparently was programed successfully):
avrdude: verifying ... avrdude: verification error, first mismatch at byte 0x0000 0x19 != 0x00 avrdude: verification error; content mismatch
Now every time I try to program it, I get:
avrdude: warning: cannot set sck period. please check for usbasp firmware update. avrdude: error: programm enable: target doesn't answer. 1 avrdude: initialization failed, rc=-1 Double check connections and try again, or use -F to override this check.
On that last unsuccessful program (the one with the verification error) I had pulled down the reset pin manually. Could that have made such harm? According to my multimeter, the reset PIN is pulled up by default (so it is supposed that it is still configured as reset).