I am a total plonker sometimes. Most of the time probably.
I bought 3 of these opto-interrupt thingies yesterday. You can see in the pic there's a plastic locating pin in one corner- I thought that was a moulding error so I snipped it off.
But unfortunately in the process of that snipping I also cut one of the connection legs off, perfectly flush with the housing. =(
Well, they do say you should buy spares....