Assume that you create lots of custom content types that are based on a base content type in Microsoft SharePoint Foundation 2010. In this situation, you cannot create another content type when the content type IDs of the custom content types have gaps between them.
To resolve this issue, apply the following hotfix update:
2536591 Description of the SharePoint Foundation 2010 hotfix package (sts-x-none.msp): June 30, 2011
Note To enable the hotfix package after you apply it, you must deactivate and reactivate the feature that includes content type definitions for each SharePoint site that uses the feature.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.