Yeah, good point! Since someone has already been convicted for being in possession of magazine parts that were not assembled.
If you have both a 9mm and a 40SW magazine, and you take them apart and put all the parts in a bag all mixed up, you could be convicted for possession of a prohibited device, if it can be demonstrated that assembling them in a particular way results in an illegal capacity magazine.
Why stop there? Simply having the two magazines in your possession could result in a conviction. Magazines come apart easily, without tools in many cases, so there's really nothing stopping you from putting the parts together to make a prohibited magazine.
This really goes to show that the firearms act and criminal code needs to be re-written.
Since a guy in BC was convicted for possession of unassembled magazine parts, what's to stop any of us from being convicted for having illegal magazines? Maybe some random items from my home can be cobbled together to make an over-capacity magazine. Hmmm... A piece of this gun, a piece of that gun, a bed-spring, some epoxy, duct tape, and voila, 30 round magazine. Off to jail for possession of parts of a prohibited device. Stupid stupid stupid. Needs fixing.