Signals returning to danger

You'll find the account of the incident on the Picc on Tubeprune's website.

Signals will, of course be legitimately returned to red by the passage of trains, although there is a lot of attention paid (principally on NR) to whether or not these are 'first wheel replace' or 'last wheel replace' - clearly the last thing a signalling system needs is a Motorman seeing a signal dropping back to red legitimately and then putting the anchors on because the signal is now at red.

Other legitimate occasions where a signal can drop back without failure and route-changing is when a train protecting a converging junction has a train just nudges over the approach track circuits on the other line - the signalling system would 'see' this as a potential conflict and put all affected conflicting signals back to danger: 'flank protection'.

Likewise if a train overuns the normal stopping mark in a terminal platform and breaks what is known as a 'track circuit interruptor' signals will be held at red - because the system will 'think' that the train has derailed. However, this isn't quite what you are asking.

If for any reason the signal operator has information that needs the approaching train to be held, the route would be cancelled, returning the signal back - though this will not necessarily mean that the actual route set is changed - just the route 'called' by the signal.

I'm sure there are plenty of other examples too, but I can't immediately think of any others.