Comment by astrobe_

Also sometimes: the bug is not in the code, its in the data.

A few times I looked for a bug like "something is not happening when it should" or "This is not the expected result", when the issue was with some config file, database records, or thing sent by a server.

For instance, particularly nasty are non-printable characters in text files that you don't see when you open the file.

"simulate the failure" is sometimes useful, actually. Ask yourself "how would I implement this behavior", maybe even do it.

Also: never reason on the absence of a specific log line. The logs can be wrong (bugged) too, sometimes. If you printf-debugging a problem around a conditional for instance, log both branches.