Error message "marked for deletion by an IMAP client"

[vertigo220]

Hovering in the blank space of the inbox I saw a message saying function(e){return-1===t.api.getNumericValue(e[0],3)?"This message is marked for deletion by an IMAP client":null} which not only should the user never see such a message, but I don't even have an IMAP client set up for this account. I saw it when the message I deleted, then moved to another folder, then moved back to the inbox is selected. The message went away when unselecting it, and reselecting it didn't cause it to come back.