I'm a Canadian and overstayed in the US by 10 days (my departure was delayed because of issues getting a new passport) 6 years ago. I departed the US voluntarily, by plane. The border official didn't scan my passport at the airport. I'm now considering a 2-month holiday in California. Could there be any issues?
You did not trigger an automatic ban. The automatic ban is triggered by more than 180 days of "unlawful presence" (8 USC 1182(a)(9)(B)), and you accrued no more than 10 days of unlawful presence.
Furthermore, you did not accrue any unlawful presence unless you were given an I-94 form specifying a date of departure. If you entered the US by land, you probably did not receive an I-94 form.
The main issue you might have is that your previous overstay could lead a US officer to deny entry, for example because of a conclusion that you can't be trusted to comply with the terms of admission.
Your presence in the US for greater than 183 days may also have made you a resident for the purpose of US tax law, in which case you might have some tax liability issues to sort out. It is, however, unlikely that the US has enough information about this for a CBP officer to be aware of it.
Since the overstay occurred six years ago, and CBP I-94 electronic records are available for the past 5 years, you may have to file a Freedom of Information request to know whether there is an issue.
You might first check your I-94 Arrival/Departure Record for your travel history. With luck, the earlier visit and departure may still be included in the results, and reassure you that an upcoming entry would be uneventful.