Lots of great entries on this one. Short of the winner, the ones that made me laugh the most (in no particular order) were:
Trouble is, after every burst we have to wheel him back from St Pancras. --Ninthace
I wanted to fly a Sopwith Camel, and all I got was the hump!! – Wensleydale
And so son, that's why it's been known as the Guard's van ever since – Traffic is Er Was.
"You've not only run over the marshaller, you've got his head and shoulders caught in your contraption." – c52
Combat was going well until the hun deployed leaves on the line. – Nutloose
Army Medics trial new methods for mass vaccination against Spanish Infuenza. MPN11
But one entry not only conjured up a improbably funny scene, but did it with style and panache (or something like that). So the coveted trophy goes to Traffic for:
Cholmondeley-Smythe loitered patiently up-sun, just waiting for a careless Hun to be pushed into range.
The trophy will be dispatched shortly -- just need to get the transatlantic rail link complete and recruit a few more pushers for the relay.