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I'm a German citizen. I was on vacation in Tijuana and I entered the US an the San Ysidro border control. I got the green I94-W form. I did some day trips to San Diego and back to Tijuana, but I completely forgot to return the I94-W form after my last visit to the US.

I flew back to Germany from Tijuana airport after a stay of 3 weeks. I still have the boarding passes of my return flight, but no further stamps in my passport. I'm planning to go to Tijuana again, but I'd like to arrive in San Diego and then go to Tijuana from there, because it's much cheaper.

Am I going to have a problem entering the US? Is it enough of a proof that I left the US in time if I show the boarding pass from my return flight to Germany departing from Tijuana airport?

The CBP help page says "If you departed by a commercial air or sea carrier (airlines or cruise ships), your departure from the U.S. can be independently verified, and it is not necessary to take any further action", but I didn't depart from the US. I'm kind of clueless now and would appreciate some help.

Thanks for your help!

Am I going to have a problem entering the US?

Maybe, but if you do it is unlikely to be a big problem.

Is it enough of a proof that I left the US in time if I show the boarding pass from my return flight to Germany departing from Tijuana airport?

Yes.

The CBP help page says "If you departed by a commercial air or sea carrier (airlines or cruise ships), your departure from the U.S. can be independently verified, and it is not necessary to take any further action"

Because you departed the US by land, the above does not apply to you.

but I didn't depart from the US

Yes you did: you crossed the land border into Mexico before you flew to Germany.

The CBP has a page for people in your situation: I still have my I-94. It contains instructions for sending the evidence of your departure to an address in Ohio. It doesn't mention that you can also present this evidence when you next present yourself for admission to the US, but you can.

On the other hand, if you show up at the border and your evidence is somehow not accepted, the negative consequences would be pretty severe, so you might as well send it to Ohio before you next try to enter the US:

If you departed by land, private vessel or private plane, you will need to take steps to correct the record. If you do not validate your timely departure from the United States, or, if you cannot reasonably prove you departed within the time frame given to you when you entered, the next time you apply for admission to the U.S., Customs and Border Protection (CBP) may conclude you remained in the U.S. beyond your authorized stay. If this happens, your visa may be subject to cancellation or you may be returned immediately to your foreign point of origin.

Before sending your evidence to Ohio, however, you should check your I-94 record online to see whether your departure has already been recorded. If so, there's no need to do anything. In your case it seems unlikely, but it can't hurt to check, and it might save you some trouble.