I've seen plenty of errors in my time, but never one like this !!

I assume from a bit of research that it's a Partial Collar whereby the collar has slipped during the triking process ?
Was noticed by my old lady (who works in a bookmakers) as it stood out in the 'stack' of 5p coins.
(I realise this is the UK section not the World Error one but assumed it would be more fitting to post here - Move if you see fit)