Less common, but always amusing:

    
>perl -wMstrict -le
"my ($x, $y) = (0, 0);
 if ($x =! $y) { print 'Huh?!?'      }
 if ($x)       { print 'What the...' }
"
Huh?!?
What the...
[download]

I almost lost my mind one night over a C code mal-expression like that. Now I always set the compiler to exclude or warn about assignment in conditionals if possible, or fall back on the
    #define is   ==
    #define isnt !=
hack if not, or both.