I've seen no such quirks myself, although when moving from Tiger to Leopard I was one of the people seeing blue screens at boot, and was very angry. Ultimately

I've seen no such quirks myself, although when moving from Tiger to Leopard I was one of the people seeing blue screens at boot, and was very angry. Ultimately, you should never expect all your applications to work on day one; if you have things that "must" keep working, wait awhile before upgrading the OS. For one thing, not all developers have advance access to the OS (like me; I only find out if my code works when I buy the retail copy of the OS). Even if a developer does have advance copies, it is possible that a tiny change between the beta and GM will have unexpected consequences. Fortunately in my case, I had to change nothing to support Snow Leopard (and I still support 10.3.9!). But not all apps will be so lucky.