Steps to reproduce:

. Go to a job's configuration page
. Set to "Build periodically", but enter something invalid ("foo") for the schedule
. Hit apply without moving the focus out of the schedule input field

Result:
An empty error dialog pops up, filling almost the entire browsing area (see screenshot). The dialog is movable, and the error message is behind it.

Notes:
(1) If you move the focus out of the input field, the error is clearly visible. Granted; but still
(2) Sorry if I'm putting this under the wrong component, can you point me to any help understanding the list of components?