sheetcam.com

I noticed that when I am trying to rotate a part and type in the angle in the "A" box, that if I make a typing error and type in "*90" instead of "-90" the program will crash.

I bet it's trying to do math with that figure. /90 probably does the same thing.

I can replicate the fault here. Most input boxes allow you to enter expressions. For instance if you want to define an operation that always cuts through your material plus a bit you could set the cut depth to: MaterialThickness + 0.5mm
Now if you change the material thickness that operation will automatically update to suit. You can see what variables are available by turning on View->Variable monitor.
If you enter something that the parser does not understand you should get a 'could not parse expression' error. It should not cause SheetCam to crash. The crash appears to be specific to the part angle.