[It sounds like your seal wasn't very secure, so I'd suggest trying to retighten the nut a bit better. And do your best not to jerk it back out of place when closing that ball valve.
If your sump is set up correctly, even if your return pump is turned off, your sump should hold the excess water.
If you are using a Durso Standpipe in your overflow box, that will limit even more water from draining down.]