Several applications fighting for a type...
I just spent a while on the phone helping a friend to figure out a problem she was having with Word on her iBook. Whenever she tried to print in Word, it complained that this wasn't a supported function. As it turned out, her iBook came with a demo of MS Office, and when she installed the copy of Office she'd bought, the demo wasn't removed. So, whenever she double-clicked an Office document, the demo would launch instead of the registered version, and the demo had printing turned off.
The lesson I learned from this is that, if I have an installer, I'll not only have it remove older versions (or at least offer to remove and warn the user). I will also make sure my demo can detect an installed copy of the final version and warn the user that the wrong version was probably launched. And I won't forget that "Don't warn me again" checkbox either.
And if you are a big software publisher whose apps suffer from this, it might be a good idea to file a bug with Apple to encourage them to provide a facility that allows marking certain versions of an application as "secondary" to another. So, the full version would be primary, the demo "secondary", and when both of them are installed, Finder would by default prefer the primary one. Also, it'd be nice if Apple introduced a more intelligent scheme for storing a version number that would allow Finder to detect beta versions and declare them secondary to their final version etc.
Filed as Bug ID #4355225. I know it's already Sunday, but it's the idea of Happy Apple Bug Friday that counts.