When I was living in Northern Ireland, I remember a time in 1991 when BBC Belfast had to evacuate the Broadcasting House in Belfast, and transmission was transferred to BBC One in London and the South-East. I remember my mum and dad were waiting for Inside Ulster to air at 5.35pm, as BBC One NI used to air Neighbours at 6.30, suddenly the picture froze and we went straight to a BBC One London/SE continuity, and into Neighbours.
We knew something was a miss here, and after the Six O'Clock News we went straight into Newsroom South East with Guy Michelmore. Baffled at this, but delighted to see a different region for once.
I remember BBC Radio Ulster aired a bulletin at 7pm, apologising for the disruption, and they were back in the building, a false alarm.
Inside Ulster was back at 9.28pm.
I remember that too. They managed to air “Inside Ulster News” with Sean Rafferty at 6:55 as normal (this update was introduced when the Six O’Clock news ending changed from 6:35 to 6:30) although it was from the main studio rather than the bulletin studio they usually used at that time.
There was another occasion where technical problems meant Inside Ulster was crashing in and out of Neighbours at 5:35 but didn’t manage to get on air until about 5:50.
Not a breakdown, but I remember that in the early 90s BBC 2 Northern Ireland used to air the BBC 2 South East programme “First Sight” complete with the South East ident at the start and trailers for South East local radio stations at the end. Not sure why, happened for a few years IIRC. The English regions opted out (it was the Thursday regional slot) as did Scotland and Wales but I think Wales aired it too, just at a different time.