If you want make a copy of multiple parts with the Mirror over a Infinite Plane with pressing Ctrl, it works not. For me, the most time i need a copy over the mirror and not just Mirroring a part.

And the Infinite Plane should stay for other parts and not for every Operation you have to select the same infinite plane... How about it stay for a while - until i select a new one? The Stay-Problem is the same with all other tools. Why can't the solid stay when i want boring some holes in it or when i wanna make several trimming operations? But this is off topic and should be discussed in another thread.