On the related topic of boarding refusal, my return routing was SFO (AA) JFK (BA) LHR LGW (BA) ALG.
The AA app wouldn't give me a mobile BP for any sectors so I feared the worst. But the AAgent at SFO didn't bat an eyelid and ran off all but the LGW-ALG on the spot no questions asked...
At JFK however, the JFK-LHR BP wouldn't scan so I was sent to the desk (with my heart in my mouth at this point
) for a "passport check". The agent clearly had a system prompt to look for an Algerian visa, which he did, even checking the dates were valid !!! Fortunately he was clearly no French speaker as he missed the bit that said "single entry only" and had a nice big immigration stamp all over it
So bit of a near miss there
and I would never attempt any ALG visit, transit or otherwise without the exact paperwork !!