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I remember a lesson with Gill where we were drawing multiple diameters in a circle. Where else would there be a diameter? (Or was it a Dia ... Meter!) Anyway, the idea was that all the lines would cross in the centre. Mine didn't so I drew a filled in circle at the centre; more of a black blob. It didn't fool Gill though and he wanted to know why I had done it. Looking back I think he found it slightly amusing although at the time I was worried.
I wonder how many more incidents that I found worrying were probably quite light-hearted if only I'd realised.