Occationally, the Sales Order Add function gets into a weird state were it will not let you go to the Line Items Screen.  To fix it, you need to exit SO-A and then got back in to continue entering orders.
 
Here is the Sequence of events that causes it to get into that state.
 
1. Click Add in SO-A
2. Enter a valid customer code
3. Press Enter till you get to RTI.
4. Set RTI to 'Y'
5. Press Enter - You should be at the Line Items Screen.
6. Click the Exit SO button.
7. Say yes to 'Cancel Adding ...' message
 
Repeat the above sequence without exiting SO-A first.  When you press Enter after setting RTI to Y, then You will be told that 'This SO is set to invoice Freight and Tax only ...'.  No matter what you say to this dialog or the following ones, you can no longer enter a new SO untill you exit SO-A.
 
This Problem exists in both the 2002.4 and the ISTECH Enhancements.