I sent the clip out to Media Encoder, and went off to do something else.
When I came back I noticed that Media Encoder was just sitting there with the spinning-ball-of-nothing going. Now sometimes it takes Media Encoder a while to get it’s act together when you send a complicated sequence to it, so I waited...
After another few minutes I started to worry.
I closed out the project in Premiere. That had no effect. I checked to see if I had too many apps running or was running low on disk space. No problem there.
I was just considering doing a Force Quit on the application - and wondering if it would remember what had been sent to it - when I remembered something: I’d opened the previous version of the output file in QuickTime Player. Sure enough, I quit QuickTime Player and … ta da! Media Encoder started encoding the file.
It’s a pity Media Encoder hadn’t just told me that it couldn’t delete the existing file.
P.S. I've since repeated this 'mistake' a couple of times. Sometimes Media Encoder recovers and starts compressing the file once you quit the other app...other times quitting doesn't fix the problem, and you have to kill Media Encoder and resend the job.