How does this happen?

[Baldwin_4-6-0]

see attached.

I fixed it be reloading the 2 links in each of the models.

[Steve_Stafford]

There are a couple threads about this and don't have the time to search right now...

You have rvt links with dwg files linked in and they (dwg's) have been changed while you were working. Revit detects this change and generates the warning. If you don't want the message you can place the dwg files that are actively changing in a "container" file and link the container into the linked project instead. The container won't be seen as changing...no error. Autodesk finally sorted out the cause over a year ago..April/May of 2008ish so there should be a Known Issue record too.

[Baldwin_4-6-0]

Thank you Steve.