Did you use the back button to get to your edit page? Have you tried manually going to the node and clicking the edit tab again to see if it reoccurs?
This message is caused when either (1) two users access the edit page and one user saves changes before the other user gets a chance or (2) when a user edits content, saves it, and clicks the back button to get back to the edit page.
Are there other users signed in? When all else fails (or rather the first step of troubleshooting always) Flush all cache. Clear your browser cache. Close your browser out. Re-open your browser. Attempt to recreate problem. Is it still happening?