>are you saying that even if you leave the vfp environment and re-enter it, the old behaviour will still resurrect itself?

If you run an application that contained a class from a classlib in a previous version, that classlib is in memory, (and its code--the old code). If you then run:
otxt = CREATEOBJECT("MyStuff") you may get an object based on the old code.

If you don't run a compiled application, then SET DEVELOPEMENT ON should guarantee that you are running the latest version of the code. A COMPILE CLASSLIB MyClassLib will ensure it.

I hardly ever compile my code into an application. If you environment is right, it's much easier to DO FORM x, or DO MainApp.PRG. It's a matter of packaging--and there's no reason to package it all in one file until you're ready to test the app as a whole.