Yes this can happen if you already have a stamp that allows you to continue to stay in Canada for a while. For example, I got a stamp allowing me 6 months as a tourist. About halfway through this period I went on a weekend trip to the US. When I returned to Canada the immigration officer did not give me a new stamp, which makes sense as he didn't have a reason to extend my stay any further. I have however gotten a new stamp in a similar situation a different time, so I guess it's up to the mood of the officer.