I have a custom field for a Review Date. In the canned Responses where you can update the custom fields, it has 12/31/1969 as a default. I cannot get this out.
So when you use a canned response in a request, unfortunately the custom field in the request ALWAYS gets updated to 12/31/1969 overwriting the date you already set it to.
This might also be happening with Request Merge feature. I have not verified this yet.