At Basingstoke, for example, you get an 'R' to go up the Reading line. If we're on a Waterloo electric service but there's an 'R' lit up on the Route Indicator we'd be sending our electric up the non-electrified diesel line! Yes the driver should check the signal aspect and any associated route/junction indicators before moving off - even when given two buzzes - but as I witnessed at a station not so long ago drivers don't always do so (train was dispatched and moved off towards a red!). The signal might be off, but if you dispatch the train up a diversionary route (in the above case an electric unit up a diesel road) and the driver doesn't notice then, well, I think you get the point.
Also got to ensure the hazard lights are all off, and even if they do all go off someone's clothing caught in a door might not prevent the lights being extinguished and interlock being created, allowing the train to move off with someone caught in the doors outside the train.
Other bits and bobs as well, but I won't bore you with those