I'm a US citizen and I visited Montreal for nine days in April. I just looked at my passport and my entry stamp incorrectly has March for the month so it looks like my trip lasted from March 9 to April 16.
It looks like US citizens are allowed to stay in Canada for 180 days without a visa, so I'm not worried about looking like I took an illegal trip. I'm just wondering if having a stamp that doesn't match any other data on me (which I assume is correct) could cause issues.