We recently had an issue when upgrading from Domino 9.0.1FP9 to 11.0.1FP2 where users had cached the old version of the Dojo/XPages framework JS.
This resulted in some pages not working correctly.
Currently it looks like the only header regarding caching that is set is Expires (10 days). If the framework JS was versioned using ETags, this wouldn't have been a problem.
Would also be nice if the same is done for framework CSS.
If you're wondering what the ETag header does: