Still have no idea why the previous sheet couldn't be formatted -- if anybody has a thought, would like to learn so I don't get in same stuck situation again.
One other possibility...were you trying to apply the Accounting formatting using a Button on the Home Tab of the Ribbon?
Those buttons don't directly modify the NumberFormat property of the cell, but instead they apply Cell Styles ("Currency", "Percent" and "Comma") to the cell. That typically gives the same appearance as if the Number Format were changed through the Format Number dialog; however it's possible to modify Cell Styles to have different properties for Number Format, Font, Color, etc...
So it's possible to modify the "Currency" Style (which has the tooltip "Accounting Number Format") to display "Text" NumberFormat.
Since you said the other sheets in the same workbook worked fine, then this possibility is a bit of a longshot.
It's possible that you used a Button on the Ribbon on the problem worksheet and used the Number Format dialog or a different Style Button on the other sheets.
Thought this was worth noting for others who find this thread when having trouble applying Accounting NumberFormat through their entire workbook.