It happened to me a couple of weeks ago. It has likely happened to you, or will happen. You order a certificate and when it arrives find it isn't the person you hoped. I was chasing Jennie Margaret Child who was born in Carmarthen in 1903, appears in the 1911 census in the home of my grandaunt in Chorley, Lancashire, then disappears.
I thought I'd found her marriage in 1935 but when the much anticipated certificate arrived it turned out to be another Jennie Child.
The other thing to do is add the information to a postum at FreeBMD which is what I've done. You can do that even if the certificate isn't unwanted.
Thanks to Christine Jackson for the tip about the Certificate Exchange.