Strict mode was recently implemented for AAO. Strict mode will cause the player to interpret bad practice as errors. Since bad practice can be as innocuous as a missing var, code that worked fine previously may no longer run.
Unas's description as to current testing:
For the player, I basically played through the Bright Age of the Law trailer (which uses many features of the display engine, as it was meant to showcase V6) and my own old never-finished trial (which made use of variables, investigations and CEs).
For the editor, I tested adding and removing elements in each tab, and try opening pretty much all panels I could find and create investigation and CE blocks and so on.
Parts of the code which were maybe not tested extensively enough are form elements, player actions, expression engine..
All features should be tested before the next release. This may be a multi-person project.