Running /save-all flush puts untranslated messages in the chat or in the server console.
To reproduce, simply run /save-all flush and look at the chat.
This seems to happen because it attempts to print commands.save.flushStart and commands.save.flushEnd when necessary, but those messages simply don't exist (while the other save-related messages do).
These strings were removed in 13w16a (yes, 13) but were present in 1.5.2. The original values were:
It isn't clear why they were removed, since the flush argument was still present then (as it is now).
Found while attempting to reproduce
MC-106551, not that it matters.