When accessing a revision hidden by a user with admin rights, with the edit text deleted it is possible to display the contents of the hidden revision thanks to a comment box on the diff screen, with a clickable link displaying the text.
This feature is unavailable when edit summary has been hidden, so there is no fast way of checking what the hidden edit summary contained.
Version: 1.25-git
Severity: minor
Sounds like revision delete.
There is a link "(change visibility)" nearby the summary and on that page you can see the summary (striked through) or add "&unhide=1" to the link. If there is no link, that summary maybe is suppressed (deleted by an oversighter).
Needs testing, if there should be a link with &unhide=1 in that interface along with the change visibility link (for example for users which can only see the content, but not revision delete any parts)
Note that said "comment box" appears instead of the "diff screen" when the revision text is hidden. As a sort of workaround for "checking what the hidden edit summary contained", you can click the "(change visibility)" link, which will display hidden usernames and summaries too (this can also be used for accessing deleted log entries).