It happens sometimes. Where I work, it's not infrequent that some weird bug will come up (QA will find it or end users will report it or whatever) and we start looking into it, but never find the issue. The resolution ends up being "hopefully it was just a solar flare." (By which we mean "hopefully it won't ever happen again and we won't have to invest more time in trying to figure out why it happened.") And often it is.