I have a certificate where there is no father entered for the bride -- just a line through that section so i though she was probably illegitimate -- until i found her living with her father upto the census before her marriage and her was still alive at the census afterwards too!
He may have been the mother's husband, but was he the father?
The entry is just what was volunteered. No documentary evidence was needed.
I have a man who was illegitimate but the mother married the father soon after the birth (there's a matching Bastardy Bond). He married with his birth name (father left blank) but lived all his life with his father's surname. His children were all registered with his birth name too. It was very satisfying to work it all out!
I have marriages with father invented, father "deceased" but alive elsewhere, and in one very lucky case, father named even though he had left the district around the time of my gg gm's birth.