Users often disregard these one-time messages and select the “Do Not Display This Warning in the Future” check box. See example below.
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The warning message displayed when you use an item on a purchase form that is mapped to an Income account only.
To enable these messages, click Edit > Preferences > General and select the Bring Back All One Time Messages check box.
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To be notified of transaction errors previously disregarded, select the Bring Back All One Time Messages check box.
From Laura Madeira’s QuickBooks 2010 Solutions Guide