Recently a client has been having issue with page layouts that contain a UserField control (People Picker). Sometimes, the People Picker will loose the resolution of the user selected and instead display a bunch of strange XML/XSL along with the message 'You are only allowed to enter one item', as shown below:

As you can see, there’s also a reference to the css class ‘ms-entity-resolved’ in the XSL.

It turns out that the issue is due to a conflict between the Content Editor Web Part and the People Picker.

The path to duplicate the issue in a clean environment is as follows:

1) Create a new page layout containing a single web part zone and add the Contact field to the layout, which adds the People Picker to the page.
2) Create a page using the layout, go into edit mode, and select a user in the People Picker.
3) Add a Content Editor Web Part to the web part zone, and modify the part.
4) Open the Rich Text Editor, add some content, click OK which will refresh the page.

After the page refreshes, the user selected in the People Picker shows like it does in the screen shot above.

Here are some links I’ve found with other people who have seen a similar behavior.

MOSS 2007 – SP2 / April CU Publishing Bug (Team Eli’s blog)
Problem adding alerts after installing SharePoint Service Pack 2 (Bamboo Nation)
Issue with Content Editor Web Part and UserField (MSDN SharePoint forums)

Looking at the posts, it seems this issue was introduced with Service Pack 2 for SharePoint. As of the December 2009 Cumulative Update (CU), however, it’s still an issue. Luckily, it looks like the February 2010 CU fixes it.