Consider the following scenario:
A custom list has a custom lookup field that references another custom list that contains more than 20 items in Microsoft SharePoint Foundation 2010.
You add an item to the second list.
You set the value of the lookup field to "None."
You try to save the list.
In this scenario, the list items are not saved, and you receive the following error message:
The list item could not be inserted or updated because invalid lookup values were found for the following field(s) in the list: [name].
A custom list contains more than one field that references another list that contain more than 20 items in Microsoft SharePoint Foundation 2010.
In this scenario, only the first lookup field drop down lookup field will render correctly. The second (or any subsequent) lookup field will render in the location of the first dropdown.
To resolve this issue, apply the following hotfix package:
2687339 Description of the SharePoint Foundation 2010 hotfix package (Wss-x-none.msp): August 28, 2012 After you apply the hotfix package, you must create the following flag in the NewForm.aspx page and in the EditForm.aspx page by using SharePoint Designer:
After you apply the hotfix package, using SharePoint Designer, you may update the web part properties on the List View web part on NewForm.aspx and EditForm.aspx. To resolve the issue, look under miscellaneous and select the "UseStandardControl" check box for both web parts.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.