Elections Bias from 2007
Followers of Birmingham Politics know that in some polling stations there have been presiding officers campaigning for the Labour Party. This is not common, the vast majority are OK, but it does happen and it is a criminal offence.
For example for one of the presiding officers or voting clerks to put poll cards in the box photographed below (which is for Labour, but this is concealed) is a criminal offence. That, however, did not stop one of them doing this whilst I was present.