If you are not sure whether you have found a bug, here are some
guidelines:
If a program gets a fatal signal, for any input whatsoever, that
is a bug. Reliable programs never crash.
If a program produces an invalid font, for any input whatsoever, that is
a bug--unless the program reported a fatal error that forced it to quit
prematurely. For example, if Metafont gives errors reading the output
of bzrto -mf, that is a bug. You can run the TeX utility
programs GFtype and TFtoPL to check the validity of a GF or TFM file.
If a program gives an error message for valid input, that is a bug.
Similarly, if a program gives a fatal error when it could continue
processing, that is a bug. It is sometimes hard to tell if these
conditions obtain; you must use your best judgement.
If a program does not give an error message for invalid input, that is a bug.