In particular, when the client first opens there are a number of screens in a tab control. The first screen loads. If the user clicks the tabs in quick succession the screens open and get swapped out by the next screen. If one of those screen calls a procedure that runs in the background for a “while”, the screen can be swapped out before it returns. And this error happens.
I think I have coded around it or redesigned it so that it doesn’t happen anymore because I haven’t seen it in a while.