I have a few, but this one turns out to be a huge time waster. The habitual part is that for some reason this error is completely invisible to me until my eyes are bloodshot and I've banged my head on the keyboard several times.

my %hash = (abc => 1, def => 2);

$key = 'abc';

$value = $hash{key};

# now try to use $value for something
[download]

Maybe confession wil help.