I have had this happen a few times, so it is clearly an unwanted feature.
1\ Setup a merge.
2\ Change mind about the merge.
3\ Hit the CANCEL button at the bottom of the page
4\ Get this error
5\ Hit MERGE button to try and get away, hit CANCEL on the merge page and it says:
6\ Throw holy water onto computer and run away.
6\ Used the REMOVE SELECTED ENTITIES button
Finally it goes away. yay!
I have seen variations of this around for a while. Can’t say when it first appeared, but been a month or more I guess. Last time I went to report it it fixed itself…
Vivaldi browser, random selection of userscripts, details available on request.