I created a custom field that looked up AD users. The field was assigned to a content type and was a mandatory field.
The requirement was to add multiple people to the column so i changed the column to allow multiple selections.
Once I changed this field I noticed that when creating a document and subsequently saving the document adding content type specific metadata in Office 2003 the custom lookup column was no longer available. This caused an additional problem as I could not check the document in as the mandatory custom column required entering. I had to manually edit the document properties adding the custom field data before being able to check in.
I have no problem doing this however I am a SharePoint Admin and this is an end user type activity so pretty poor user experience.
The only solution to get round this is to upgrade to Office 2007!