No, there is a condition called 'pocket plums' where the fruit takes on a peculiar shape. No-one is quite sure of the cause, but it is probably something to do with poor pollination. Next year it may not happen at all.

According to the RHS, pocket plum is caused by a fungus (Tapbrina pruni) infection, and it seems to be capable of overwintering on twigs on the tree. Spores are carried in the air or by water splash. There is no treatment available, but they say you should remove affected fruits as soon as possible, and preferably before they go white.