I have been working on a Knapp Daneben Tapa, where each clue is wrong by exactly one unit. I have tried it a few times and keep running into the same problem. I think I am solving it deductively/not making any guesses, but I wind up with a clue not being satisfied. I am including the puzzle below. I am not sure how crediting people for puzzle creation really works (I'm new to the site), but including a link to where I got it from as well. I am looking for either a solution/some steps for comparison with what I did, or confirmation that I am indeed running into a real contradiction. I got the puzzle from the following link: http://oapc.wpc2009.org/archive.php?id=52
Agreed, there is something wrong.
I arrived straightforwardly at the following situation:
and there is no way to complete the top.
For the record, here are some intermediate situations:
Note: you complete the 7 by seeing an immediate contradiction if the other choice is made. The next step after this last picture is to see that exactly one of the two empty squares [lying above the inverted C at the bottom] has to be filled (at least one for connectedness and not two because of the 2x2 square rule), and there is an immediate contradiction if you fill in the other one.
This puzzle is indeed broken, and has no solutions. (I ran into the same contradiction as Arnaud.)
The organizer, Serkan Yürekli, made a post on the Logic Masters Deutschland forum around that time, confirming this. The 1,1,3 clue at the top was the problem, and because of this they ignored the top arrow when checking solution keys.
(In addition, the Tapa ? from that contest has multiple solutions - so if you're going through the archives, you may want to keep that in mind.)