Most common was ammended data which occured after the BT's had been created, another was whilst the BT's were being created, the parish priest or clerk did not copy data correctly.

I can highlight one such mistake for a Thomas HUTSON bapt 1753 son of John & Sarah abode Redbrook. This was copied to the BT's as Thomas ASTON and therefore is also on IGI, as LDS used for their extraction process the BT's. Unknown to many people this will have affected their research.

Baptism of a son or daughter, where father or mother given same name. 2 examples of such a mistake;
1782 Sarah Aston dau of Thomas & Sarah mother should be Anne
1815 William Aston son of William & Honor father should be Richard.

This type of error occurred in all registers at some time. The advanced search option can help to highlight such occurances.

good luck............Gill