value of error 1926 farthing

I have just noticed this raised shaped around britannia on the reverse which looks the same shape as george v head in reverse. Is there a name for this type of error ? also what would the value be please.

just found out what it is, maunfacturing problem causing ghosting on the reverse.

You've found only one like this! On the pennies it is especially prevalent. I think it is also called a halo.