Two issues that exist:
1) If WebDAV client has lock for resource and attempts a PROPPATCH, it will fail. Problem is caused by resource being locked but lock not being checked against client's lock uuid.
2) If PROPPATCH is called with multiple custom properties under <D:prop>, it will fail because of unnecessary constraint.
On Vista/Windows 7, this appears to be a problem that shows up as:
"Error 0x80070021: The process cannot access the file because another process has locked a portion of the file."