If you select and drag/drop a large number of files into the dialog and, for some reason those files are unreadable (eg the drive is no longer mounted) you get uninterruptable "<filename> could not be added to the list because it does not exist." errors, one after another. You cannot interrupt this, so if there are 100+ files you were adding you get 100 error messages one after another that you have to manually click OK on to go past.

Other, more mature, applications notice this behavior and give an option to abort the entire process if there are multiple errors, but not Acrobat, where we are forced to Kill the process.