In the days when the North London ran through to North Woolwich, services which had started in the 1980s as 3rd rail throughout later used to change four times along the way, there being two separate overhead sections which had replaced DC along the way. Can't recall the exact stations, but if you were sat under the pantograph there was a periodic considerable bang from the circuit breaker as it went up/down.
From Stratford - Richmond it was:
Hackney Wick D.C. - AC
Dalston Kingsland AC - DC
Camden Rd D.C. - AC
Acton Cemtral AC - DC
Hackney Wick to Dalston was dual electrified until 2010, but the units swapped over as they were limited to notch 2 on DC power to limit return current issues (the track circuits there were very susceptible to the D.C. return current).
Certainly back in 313 days, DC trains arriving at both Euston and Watford would occasionally change over and return empty to depot on the AC lines (at Watford using that stub on the Down Fast line). It was described in Modern Railways at the time. Does it still happen?
I remember seeing a 313 do that a long time ago at Watford, but I’d be surprised if it happened now - the fast lines are too busy. A couple of empty Services go to/from Euston on AC via the slow lines at the book ends of the day.