UUIDs (Universally Unique IDentifier), also known as GUIDs (Globally Unique IDentifier), according to RFC 4122, are identifiers with a certain uniqueness guarantee. 

Systems working with compliant UUIDs may choose not to accept randomly generated ones, and many open source validators will actually check for a valid structure.

An UUID must have this format:

xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx

Where the M and N positions may only have certain values. At this time, the only valid values for M are 1, 2, 3, 4 and 5, so randomly generating that position would make most results unacceptable.