NX 11.0.1.mp01 | SW 2016 | TcUA 11.2.3
To add to @RandyEllsworth's reply, this can happen if a user imports a part from the OS into the Named References of an existing Item Revision's UGMASTER or UGPART Dataset. It can also happen if a user imports a part into Teamcenter, into an existing Item Revision's Dataset, by either using the NX Import Assembly into Teamcenter UI or one of the command line import utilities using the Overwrite action.
In both cases, the physical NX part file is replaced in Teamcenter. Parent assemblies find the child Item ID's, however, the NX part file found in the datasets has a different internal NX UID, hence the warning.
You can turn off this message in NX and accept the imported part by checking the "Allow Replacements" checkbox in the Load Behavior section of the NX Assembly Load Options.