This is an old subject, but it helps me to understand my coin. And it should answer the pending question. It's never too late.
I have a coin like this one. The original post doesn't show the obverse, but I'm quite sure we could find a lump similar to my coin, just under the Queen's chin.
I was never sure if the lump is a bubble or produced by clashed dies with foreign object, although this last one was my preferred explanation.
The lump shape looks like a gas bubble, but in this coin the observer/reverse lumps are perfectly aligned and with the same shape.
So, now I know it exists at least a second coin with that error and confirms that the dies have been damaged. There're probably many others out there.
The hanging 2 confirms that dies clashed together. And the aligned lumps tell us that a hard floating object was in the striking chamber during that clash, damaging both dies in the same way.
Thanks for your comments and confirmation.