Glad someone like bplus is seeing eye to eye with me on this. Anyway, if it helps get closer to the problem, I can say a few things about the conditions leading to when it happened.
Besides the obvious (Win10, qb64v1.4 candidate, no AV, no, FW)
0. "Failure to close", to me right now, is defined as the program ending properly, getting the "Press any key" message, but then the X button and alt+f4 each fail to dismiss the window.
1. I was in the edit > run > edit > run etc. phase of finishing a program at each occurrence.
2. I never, or very rarely at least, have multiple instances of Qb64 open. Nor do I have the source open in another editor. No exceptions to this were going on when the programs failed to close.
3. Item 2 extends to my whole habit on the computer. Everything is closed, including browers and tabs, daily. I probably had Chrome open, maybe a folder or two, when it happened.
4. I havent made any changes to my system whatsoever beyond view settings when I bought the thing.