Canopy.
An old pal of mine, now deceased, flew every Mk of Spit right through the war. He specifically said, as Danny stated above, that for t/o & landing, the door was to be half-latched with the canopy back. The reason was that the half-latched door physically prevented the canopy from flying forward in a mishap and trapping the pilot. Having found myself thus inverted on terra-firma (Similar canopy, different a/c type.) with the canopy closed and jammed, with a red-hot engine still ticking away, I can confirm that this is highly undesirable. Those original SOP's were there for good reason. :-/