When developing an XPages application of beyond-minimal complexity, you're likely to run into a problem where your app starts saying that a class is incompatible with itself in one way or another. The exception usually traces down to something like "foo.SomeClass is incompatible with foo.SomeClass" or "cannot assign instance of foo.SomeClass to field X..." where the field is that same class. This has cropped up since time immemorial.
It's actually, though, something that IBM sort-of fixed in 8.5.3 by adding an xsp.properties option of
xsp.application.forcefullrefresh=true and then, in 9.0, a GUI option in Xsp Properties:
Basically, this checkbox amounts to "don't break my app periodically". From what I gather, the default behavior is in the interest of being clever with classloaders, but can lead to creeping problems in complicated apps, to the point where changing seemingly-innocuous things like the ACL breaks the app until you restart HTTP or "kick" the app by modifying certain design elements (namely, Java classes or faces-config.xml). Since that behavior is never desired, there is no reason to not check this box, and I enable it on every new NSF I create.