We've had an irritating issue recently with our accounting system, when creating the custom invoices.
Basically, every time we tried to create a custom invoice with Microsoft Office Accounting the entire application would crash for no obvious reason.
Checking the exception's stack trace it would talk about get_item which lead me down one path but the answer was actually (and irritatingly) more simple than that -we had a couple of merged cells in the footer of the totals table. Silly I know but it made all the difference!
So if you're getting a crash with Microsoft Office Accounting when creating your invoices, check for merged cells first!