[lhorie]

Another thing worth noting:

When you show up at a land border, you technically exit the US. Likewise, when CBSA turns you back, you'd be re-entering the US and going through US CBP. So for purposes of updating a US visas, the current ban is not relevant if you present at a land border.

[nrmitchi]

That's technically true, but in the future I believe it also means that you have officially been denied entry to Canada, which could come back to bite you in the future (as silly as it is).

It also runs the risk of Canada not letting you in and turning you back, and then the US not letting you in (if they decide your status, O-1 here but often would be TN, isn't valid, or for whatever other reason), and you.... get stuck in no-mans-land? I've always legitimately wondered what happens in that situation.

If you do happen to be a citizen or permanent resident of either Canada or Mexico, it would be safest to do your border-trip at your home-country border.

_Update_: Did some real quick research, and it appears that for US/Canada at least there are agreements in place that if one country rejects your entrance (at a land border) from the other country, the other country must take you back, but could find you inadmissible and put you in immigration custody pending removal. Fun times.