Its an AFMC(Association of Faculties of Medicine of Canada) rule, and not widely published. Its mentioned on a few Medical schools visiting student elective pages for the individual schools. It's an outstanding question about whether or not this ruling covers non-university affiliated electives. You can always ask the AFMC.
You might be able to claim ignorance, but when it comes down to your CaRMS applications you need to list all of your electives as part of your paper application (the part that is visible to all the schools you're applying too). So there's some theoretical risk that a program (and all universities to which you'd be applying are part of the AFMC) will notice that you broke the rules and use it as a reason to toss out your residency application. To get around this you could leave off one of your electives, but that might limit the utility of doing the elective in the first place.
Last year, no one I knew got into trouble for doing more; but no one went over by more than two weeks. I endend up doing 13 weeks but my excuse was that it was over two calendar years. No one asked anyway.
