When an upload finished, many of my friends / contractors didn't know you'd need to refresh the page to view files. I'd sometimes find two or three copies of the same file in a given drop.
My drop had an admin password, but also a manager password (which were different, but would presumably be used by the same person).
Even browsing for files to upload was more difficult than it should have been. Once a file was selected some users assumed it was being uploaded already.
These were manageable interface problems for me, but required delineated instructions to the "average user."