Whenever I'm debugging a problem, and I have no idea what's going wrong, my first instinct is to say: Print out *all* the variables.
My coworkers used to give me strange looks, and I'd get conversations that went like this:

Me: "Hey, print the value of X"

Them: "I know what it is. It's 3."

Me: "Print it out anyway."

Them: "Fine." Output: 3

Me: "Good. Now print the value of Y..."

Repeat for many, many variables (incidentally, this often became code that logged gobs of data to a file). Eventually, we'd get to a variable where my coworker thought it was one value, and the variable was another value. "Oh, crap! I see the problem..."

This is especially necessary for languages like C where a runaway pointer or stack overflow can trash a variable from a completely different context.

Fortunately, they're starting to understand my debugging methods now :)

$"=$,,$_=q>|\p4<6 8p<M/_|<('=>
.q>.<4-KI<l|2$<6%s!<qn#F<>;$,
.=pack'N*',"@{[unpack'C*',$_]
}"for split/</;$_=$,,y[A-Z a-z]
         {}cd;print lc
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