I found the following answer for recording a land departure. "A departure will be recorded if you depart via land and re-enter the United States prior to the expiration date stamped in your passport." I wonder how the land departure could be recorded when re-entering the United States. Does it require some evidence? Please let me know it if there is any person who experienced this.
It requires some explanation but it is not unusual. It was actually my toughest CBP interview so far (at about 2 minutes of questioning, so I wouldn't worry too much). Once the guy found the Canadian border entry stamp he seemed happy about it.
No documents or other information was sought from me, just a coherent explanation of how this had happened and about my travel plans. I was then re-admitted as normal for 90 days. No one has mentioned it since then but it comes up blank in my I94W travel history.
Please see also my answer to this question: Exit stamp on passport when leaving US by bus