How do you reproduce bugs that occur sporadically?

Question

We have a bug in our application that does not occur every time and therefore we don\'t know its \"logic\". I don\'t even get it reproduced in 100 times today.

Disclaime

Answer 1

@p.marino - not enough rep to comment =/

tl;dr - build failures due to time of day

You mentioned time of day and that caught my eye. Had a bug once were someone stayed later at work on night, tried to build and commit before they left and kept getting a failure. They eventually gave up and went home. When they caught in the next morning it built fine, they committed (probably should have been more suspiscious =] ) and the build worked for everyone. A week or two later someone stayed late and had an unexpected build failure. Turns out there was a bug in the code that made any build after 7PM break >.>

We also found a bug in one seldom used corner of the project this january that caused problems marshalling between different schemas because we were not accounting for the different calendars being 0 AND 1 month based. So if no one had messed with that part of the project we wouldn't have possibly found the bug until jan. 2011

These were easier to fix than threading issues, but still interesting I think.