I lost a monk to this bug a while ago. I know I should have posted this earlier but better late than never I guess.

Summary:

1. I was in a room with my two tame cave lions like this. I'm not sure if the reconstructed image is accurate but you get the idea.
The whole level was explored I think.

2. I pressed w+numpad 6 to use the fast walk mode to move east.

3. The game froze and I started pressing different keys repeatedly to get somewhere.

4. Finally the game view disappeared and I got this error message:

Fatal error:
"Invalid target in 'GetTargetData' (-2)"

The screenshot below.





I was able to load the save file since ADoM (or ADoM Sage) made a backup in the process but the game just immediately crashed again because of the same bug and I lost the file.

I was playing the Linux version of ADoM 1.1.1 using the ADoM Sage frontend on Windows XP. I'm sure this bug wasn't due to the frontend since it seems that other people have ran into it on the regular ADoM 1.1.1.

Links to other reports of this bug on various operating systems on the old bug tracker:
Report 1
Report 2
Report 3
Report 4

By typing GetTargetData (-2) on the bug search field I was able to find 10 bug reports with an error message like this.

I think this bug is fairly difficult to reproduce but it does exist.

I'm giving this a fairly high priority as the bug seems to kick in fairly unpredictably.

EDIT: Clarified and added more stuff. Gave a priority.