The last place I worked would pull devs randomly to do QA tasks when QA got behind. Of course, since we weren't trained in the QA processes, and didn't do it all the time, it often wound up taking longer to do the testing, plus the devs would get behind in their tasks.
It's also common practice; usually caused by testing getting under-resourced, bad tooling, or longer shipping cycles (more stuff to test, more pressure to get it out).
