Entered the US by land, left by Mexican land border. Had a flight booked but didn't take it. Will I have problems entering the US again?

I am a European citizen under the visa waiver program. I flew the US from London and had to purchase a ticket out of the US to be allowed to board the plane there. Found a cheap ticket to Guatemala from Texas.

I told the border guard that I was taking that flight out of the country. But I ended up crossing the Mexican land border instead and traveled onward to Costa Rica and then back to the EU.

So there is no record of me leaving the US (Except for the Mexican stamp in my passport).

I am wondering if I would have problems going back to the US in a few years. Can't find anything like this on the CBP website.