Another annoying possibility is interrupts/concurrency. I once had a case where 

if ($x && !$x) {...}

[download]
was evaluating to true, two or three times per hour.

In reply to Re^3: This is why logic is so frustrating (was This is why Perl is so frustrating) by SuicideJunkie
in thread This is why Perl is so frustrating by eggmatters

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