|Bonus fact: At the request of the publisher, the illustrator "reluctantly" toned down the cleavage on the girl in the background.|
You should note that the error I'm talking about was only present in the first few printings, and the copy I happen to have is a later one, by which point this problem had been fixed (according to the Fighting Fantasy Wiki, it was corrected with the fifth printing). But anyway, in earlier printings, the first encoded word was misspelled... and yet the illustration is accurate on all versions! So how did the illustrator end up with the correct code when the first versions of the book didn't? (Compare this with a similar problem with a puzzle appearing in an illustration in Tomb of Nightmares, and a problem with a secret code in another Fighting Fantasy book by Martin, Night Dragon.)
The second error is, well... debatably not an error, although it is often cited as such. When you've finished rummaging around in Castle Heydrich, you need to descend to the crypt to take on the Count himself. If you fail to find a Magic Sword before doing so, then the final encounter is unwinnable. The text claims that "other things like spells and Holy Water may cause him some damage, but they cannot overcome him". However, theoretically it would be possible to wear the Count's STAMINA down to 0 using other means, although it would require quite a lot of luck with dice rolls, so it seems reasonable that Martin would not have thought of this. Even if he did, he may also have been thinking in terms of in-universe perspective that just holy water and spells would never be enough to do away with someone as powerful as the Count, that you would need something more effective, although that is just supposition on my part. And if you're daft enough to face down a vampire without a proper weapon then you deserve everything you get.
Good advice for life, there.