Ah! now we have a little admission of guilt from airfix. Looking at it objectively it doesn't really detract from the overall kit, but it shows a little about their production.
I am assuming that they had it moulded in China and not by trumpeter who was rumoured to have moulded the mossie for them.
I do watch a lot of "How it's made" TV programmes and so learn a lot about manufacturing.
Yes, there is a problem with the sprue Q mould. It was probably fixed as soon as it turned up. If the sprue feed, where the liquid hot plastic was not quite right then this would cause problems as the plastic injected into the mould would not have got to everywhere it should properly. This in turn would cause the lens effect as the plastic would not have formed into that part of the mould properly.
This would have a knock on effect to where the part meets the sprue and the connection made there. All the bouncing and movement involved with packing and transport could cause undue movement there on the canopy part as it is the only bit of the part that attaches to the sprue and could cause the stress crack.
There could have been a lot of kits packed before this problem was highlighted and fixed. This is why airfix probably have a lot of corrected sprue Q's ready for customers who complain about this.
I am not sure how long it takes styrene to cure though. I think it's a bit of truth, smoke and a couple of mirrors.
Still, despite this and the incorrect 20mm barrel I am so happy that at last the 1/24 kit has come of age with detail to rival 1/32 kits at last!
Despite a couple of drawbacks, flash and seam moulds etc (of which most, if not all kits are guilty of in some way) Airfix have done us proud. 11 out of 10 and a lot of brownie points.
Now let's have the car door version and then a ME-262!