For example, when I loaded KQ4 yesterday, I found there are four resources (picture 150, 151 and view 198, 199) that point to non-existent VOL files. And there are five pictures (2, 10, 15, 102, 115) that have bad data for pixels (using a Y value of 168 occurs a bunch of times in all those pictures). Sierra's interpreter will happily overflow the picture buffer without complaining (the stuff that gets overwritten is whatever is beyond the picture buffer in memory, which is not even allocated to the program!). So they display with no visible effects, but if you want to draw them in a new interpreter or editor, you have to account for that.
Yes, I've come across both of those issues in KQ4 already. For resources that point to non-existent VOL files, I think we can just ignore that. Clearly they're unused resources, and logic scripts must not be referring to them. Some of the other games have that issue as well.
For those pictures where the 0xF7 relative draw tool is drawing down to Y = 168, I'm not 100% sure what to do. If I remove the validation check, then they load fine and render fine in my Interpreter, but then that means we're allowing an original Sierra game to violate rules that our own editors enforce. Sierra's original PICTURE editor must have allowed them to draw down to 168, perhaps as a result of a bug maybe? Given this issue is in KQ4, maybe it was one of their later PICTURE editors. This issue doesn't seem to be in any other game that I've noticed so far.
I guess what I need to do is put some sort of option for disabling that validation into the Picture part of the AGI Library. Maybe original games can use the AGI Library with validation turned off, whereas the picture tool in Visual AGI would have to run with it turned on, although it probably doesn't need to, because if it has been written properly, it won't let such data be created.