That my passport had expired!
Visiting the UK for a few weeks to catch up with friends etc.
Drove up from home in Spain to the tunnel. Arrived last night in Calais and at the “UK border” I handed over our passports. The passport chappie asked me “do you have travel documents that are not out of date?”
He waved me through to the train and at the UK end there are no checks, so I’m in the UK now but I’m going to have to get a replacement passport before we can return to Spain because he said we won’t get past French customs with an out of date passport.
I’ve always thought only idiots would turn up at an airport or other passport control without a passport. Now I am that idiot!
Edit:
Sorry, I should add that I have a UK passport, which expired whilst I was I. Spain.
Should still be shot though, for being a muppet.