I've only crossed the US/Canada boarder by land twice. Both times going from Vancouver to Seattle on Amtrak services. 1 bus, 1 train. Only 'issue' I had was at the boarder station when on the bus, and that was more because I had more that I had to do inside then everyone else did (having to go through the US-VISIT and I-94W process while everyone else on the bus didn't).
I had more problems entering Canada at YVR.
Most annoying times I've had with CBP was once at BOS and once at JFK.
At BOS arriving from AMS via LHR as part of RTW holiday. First stop in US was attending a friends wedding. CBP agent was going up the line ahead of the booths checking people had paperwork ready. That agent demanded to see the wedding invitation, which I had when packing in AMS, but couldn't find in my carry ons. Said it was in my checked bag. I was pulled to the front of the line and the agent went and said something to the next booth agent. I went through the same thing with that agent and was sent to secondary.
The agents in secondary got pissed off at the primary agents for sending me there for no reason and wasting everyone's time.
The time at JFK, arriving from HKG. Agent wanted to see my ticket, again a RTW trip. Ticket had been issued in Tokyo and was priced in Yen. Agent saw the ticket price, and for some reason was unable to understand was JPY meant and assume the amount was USD, then assumed I couldn't afford my holiday. Spent the next 10 minutes questioning my finances in different ways before letting me pass when "credit card" was mentioned.