The windows is indeed of type Response!, so it's not just a side window of some kind.
As far as what's happening, it's almost impossible to tell internally. The window has a grid DW on the left, and a detail pain on the right. On rowfocuschanged it posts an event to check the data on the row and populate a text box with a list of possible effects based on existing data compared with the new row. The grid has a couple buttons (so buttonchanged occurs too) and can mark rows done or unused. The whole process works most of the time, and appears to die at random (users can't put a finger on specific data or sequence of events that are killing it).
We have other windows (and response windows) with much more complex code & logic on them, and there's no problem.
We've checked all the referenced columns in the datawindows, nothing is spelled wrong or missing (the datawindow objects are static and don't change at runtime).
I tried to build the app using machine code to see if it would crash at runtime, but the build failed for unknown reasons (invalid C code got generated, and crashed the C compiler). Can anyone think of why the following line of code causes a crash?
decimal{2} qty[]
qty[1] = 3.5
if ceiling(qty[1]) <> qty[1] then error++
The if statement fails to compile to C.