Click Preview twice and alert message insinuates that the wiki page is being concurrently edited by someone else (BB-2726)
The full alert message is:
Oops, it seems that this page is being concurrently edited by someone else and we don't perform automatic merging. If concurrent editing is important for you, consider cloning the wiki repo and working offline.
This behaviour is new, unexpected and annoying as I am then unable to Preview the wiki page more than once, making the Preview feature completely useless.