Well, I would say that the BBC do this quite frequently, putting the wrong region's news and weather bulletins on. In fact, BBC Midlands had about 15 seconds of BBC London news tonight at 6.10pm before Midlands Today started from the start - no apology and no overun/cutting off at the end.
But I've never seen this happen on ITV before - so fair point, this does deserve a thread - as did 'BBC Two/Lambing Live Problem' - another recently popular topic!
Well that's because Midlands Today control when and when not they 'opt', London is (I believe) the sustaining, network feed, so without an opt you would just get BBC London.
These mistakes were most common a few years back when Suzanne and Nick would thank the network presenter(s) before continuing, they'd show
N6 on the projector screen or plasma. Most famously was when they did this and had BBC London crop up on the plasma, just the logo, went to the headlines and ended up with Rizla-paper teeth reading her headlines and the BBC London titles.
Whereas on ITV, ITV Central is played out at the Northern Transmission Centre in Leeds and there is no 'network sustain feed' as it were, just black. During Daybreak you'd get the purple Daybreak logo on the purple background 'property' with "Continues shortly" and "ITV Central" on it, the region overlaid by Leeds as a fall back.
As an aside, as it is the Southern-most NTC region it has to provide bulletins to the STC regions if the STC goes down, like 2009 when they used ITV Central for Central and the STC regions.