So today I was creating a profile using the 1939 register at Findmypast, to flesh out the biography. As usual, I inspected the original image and found that the transcriber had turned a Kathleen (female) into a Keith Ian (male), with a married surname (which really should have been a clue to the transcriber, re. the sex of the person). This is the first major error I have found on the 1939 register, although Findmypast also have my great grandma married to the wrong bloke (making him a bigamist on paper) and I've found mistranscriptions by the bucketload on censuses.
I frequently read people's posts on here that say, often quite proudly, that they only ever use free sources (most of which are transcriptions of images you can only access by subscribing), but the fact of the matter is, whether you pay or not, if you haven't seen a clear image of the original, you don't know how accurate the transcription is. I know not everything is available in image form, but in the interests of accuracy, if it is, we should use it.
Imagine if I had not seen my GGM on the census, or had conversations with my mother about her family, I might have been off on a wild goose-chase back to the 17th century or earlier on a family name that is totally irrelevant.
I personally feel that the ~£2.50 a week I spend on Findmypast is money well spent, but my point is this - even with paid sourcing, it is evident that you need to stay vigilant and if something doesn't look right, look into it or find another source that confirms/refutes.