Compatibility Step 5
One tricky difference between QuiteBASIC and the real old classic BASIC dialects were that QuiteBASIC were only allowing brackets to enclose array indices, like so: A.
As of today, QuiteBASIC will also allow parentheses like this: A(5).
This was one of the big remaining incompatibilities. The few most important remaining issues are multidimensional arrays and the (ugly) "ON" command.
I keep track of incompatibilities on this page in the club house.