When I was debugging with multiple projects open today, the debugger got confused about who owns what breakpoints (really, I think it was QtCreator setting breakpoints wrong.)
To replicate open multiple projects and set some break points in one.
Switch to (make active) the other project and debug that (without breakpoints)
Somehow, even though the current project doesn't have any breakpoints set, it will break on lines in files corresponding to the former no-longer active project.