5p Genuine Error, Or Not?

Welcome to the forum.

I have noticed in recent years that the production quality of the lower value UK coins does not seem what it once was. These blobs or Cuds are errors but not particularly valuable, as they are fairly common.

Your coin also seems to be stamped slightly off-centre on the reverse judging by the thickness of the rim. But this might only appear this way because you have taken the picture at a slight angle. These slightly off centre coins are also not particularly rare. I have seen quite a few of these - I am an "anorak", checking every coin that goes through my hands ( and coins going through other people's hands if they will let me...). If you find a coin with the design so off-centre that part of it is missing, then you have something worth a bit.

regards