Well, I spent ages debugging a PHP script a while ago. (No PHP is not my usual language :) )
I found that if using a variable that wasn't defined,
if($undefed_var == TRUE)
{ // Never gets here... }
else if($undefed_var == FALSE)
{ // Never gets here either... }
else
{ // We end up here! }
You don't know how long this took me to work out, since the reason the
variable had not been defined was because of a typo in its name :)