You've missed my fundamental points.
There is a qualitative difference between a mistake and a lie. Different aspects of a narrative are more important or less important than others when considering its veracity.
I will make this directly relevant to a hypothetical photo of the LNM (not this particular one) although the principle would apply to any other Fortean subject:
1) A witness says, "I took this photo of the LNM on a family holiday in 1990."
Investigation shows that a building in the background was demolished in 1987, or, alternatively, there is a 1992 registered car in the background.
Investigators challenge the witness who says, "I went on family holidays in the area over a 10 year period. I must have been wrong about the year."
All this proves is that the witness has made an honest mistake over a peripheral detail. Their evidence is neither more nor less reliable than a typical honest person's recollection of events 20–30 years ago.
2) A witness says, "I took this photo of the LNM on my honeymoon. I remember it well. I got married in 1990."
Investigation then reveals evidence that it cannot possibly have been taken in 1990.
The witness has said something very specific, giving a definite reason for their certainty. However, this has since been shown to be untrue.
Have they lied or made a mistake? The onus is on them to explain why they were so certain of something that is now discredited. If they cannot do so, I would consider their evidence to be less reliable than average.
I would treat all of their testimony with caution until this had been resolved. For example, they may be able to establish that they also went on a trip to the area with their fiancée 2 years before they got married, or that they went back to celebrate their 5th anniversary. If so, I would accept that they had mixed up two broadly similar life events.
3) A witness says, I clearly remember taking this photo of the LNM. It was 1968. I was a student and went on a fishing trip with my friends. I was fishing by the loch side when I saw some ripples. I stood and watched for a minute or two then remembered my camera was in my rucksack. I took several pictures.
Here, they have given a very detailed account of the circumstances in which they took the picture, something that would have been very memorable.
The witness then submits only one photo. Investigation shows that the photo cannot possibly have been taken from the loch side as it must have been taken from much higher up. Therefore, something that is fundamental to their narrative is untrue in a way that cannot be "explained away". If they have lied about this, then this casts fundamental doubt on their reliability as a witness and as such, all of their uncorroborated testimony is unreliable.
- I would put the post that started this thread in category 3: a detailed account with a fundamental aspect which has not stood up to investigation.
None of this proves that the LNM does not exist. It only affects how I would assess this particular piece of evidence for its existence.
In the insurance scenario, a person who simply misremembers a date or a price, within reasonable limits, does not invalidate his claim. However, a person who is caught out telling an outright lie about one item will usually find that all of the unproven parts of their claim are rejected or put on hold. In the worst case, their policy may be made void and they will then lose their entire claim.
I used to attend civil court regularly at an earlier stage in my career when I was dealing with motor liability. If a Judge identified one obvious falsehood in a witness's statement, they would often dismiss all of their testimony. If they only identified minor inconsistencies, they would usually just factor this into their decision, saying something like, "No one has come here to lie to the court, but Witness A's account is less reliable than Witness B's."
[Slight edit made under point 3 for clarity. No change to meaning.]