The most common variable error you're going to get, most likely, is where you typo'd the variable name, in one way or another. For example:
Flea=0woot=0thatotherobjectoverthere.ex=0pwnt=flea+wootr-ex
The above code, would throw an error if put into a completely blank GM game. Capitalization, spelling, and what object it is, are all important, and should be looked over carefully.
As for the unknown variable error, simply go to Global Game Settings, Errors, and check "Treat uninitialized variables as 0". This will fix that particular error, though you can still get bugs relating to variables. The above quote was also an example of an unknown variable error causer.