I live about 2 exits away from there, been here about 10 years. It sounds silly if you've never been near there, but any single fuckin one of you could make the same mistake.

If you stay on the I-5 south it'll announce with large signs that says "Last US EXIT" - now if you miss that, then you're an idiot. Also, there is no line exiting the country, only on the way back in.

What probably happened to this guy was he was going horizontally on the map at some point, along the border town. There, it may just say I-5 South for an on ramp, let's say you take that and you were already at the last exit, it's funneled into the port of exit, no room to turn around unless you stop in the middle of the freeway (which will probably get you booked anyways).

It's happened to US Citizens plenty as well, one of my friends got lost and ended up in this situation, basically got x-rayed on the way back with like a 3 hour wait + 2 interviews (he's white/citizen btw).

Basically as swim said, use caution if you're near the border and unfamiliar with the area.