Hard to say. I assume you're using the Domain Aggregate version (the first version with DMax). The Nz function is trapping for null, plus 3 of the 4 fields in the test table are using autonumber or default values. Does your "real" table already contain any Null values? Are the defaults set correctly? Are the data types and field names identical to the test table where appropriate?
If the On Current event is causing the error, you need to figure out which line of code is raising it. Open up your code module in the VB
Editor and click anywhere in the line that reads 'Private Sub Form_Current()'. With your cursor in this line, press the F9 key. A reddish highlight will appear around the procedure header and you'll see a red dot in the left margin. This is called a break point. When you open your form again, code exectuion will stop on this line of code and the highlight will be yellow. You can then step through the module (slowly), line by line, by pressing the F8 key. Each line of code executes as you press the F8 key and the yellow highlighting moves to the next line of code. Hovering your mouse pointer over any variable or control names while in break mode will display an Auto Data Tip box telling you what the current value of the variable or conrol is. Check your variables and controls as you go. Do you see any Null values? Which line of code raises an error? You can remove the break point by pressing Ctrl-Shift-F9.
I'm going to go watch the Yankees loose now, but I'll check back latter.