If the computer gives a wrong answer (verify that the answer is wrong!) or crashes, it is a bug. If any one program crashes or gives an operating system error message, it is a bug.
If a command never finishes running can be a bug, but you must make sure that you didn't tell it to take a long time doing whatever you wanted it to do. Ask for assistance if you didn't know what the command did.
Some messages will alert you of bugs. Some messages are not bugs.
Check Section 
and any other documentation to
make sure they aren't normal informational messages. For instance,
messages like ``disk full'' or ``lp0 on fire'' aren't software
problems, but something wrong with your hardware--not enough disk
space, or a bad printer.
If you can't find anything about a program, it is a bug in the
documentation, and you should contact the author of that program and
offer to write it yourself. If something is incorrect in existing
documentation
, it is a bug with that
manual. If something appears incomplete or unclear in the manual, that
is a bug.
If you can't beat gnuchess at chess, it is a flaw with your chess algorithm, but not necessarily a bug with your brain.