Uninitialized variable (error 48)

There is a "hidden" error (not listed in the help menu) Uninitialized variable This means that a return value wasn't specified in a function:

DEF EXAMPLE(EX)
 IF EX==1 THEN RETURN 1
END

Looks OK, but if EX isn't 1, the function will reach END and have nothing to return. IT should be changed to

DEF EXAMPLE(EX)
 IF EX==1 THEN RETURN 1 ELSE RETURN 0
END

And also:

DEF EXAMPLE EX OUT EX2
 IF EX==1 THEN EX2 = 1
END

should be

DEF EXAMPLE EX OUT EX2
 IF EX==1 THEN EX2 = 1 ELSE EX2 = 0
END

This error can also happen when you make a typo like:

VARIABLE=