This might be obvious, but I just wasted a few hours because I failed to turn off MaCafee before installing the BootCamp update. The machine just kept hanging in the middle of the install and terminating the install abnormally set up a whole chain of installer failure events requiring me to restore my entire Windows partition from a backup.
It was all because MaCafee was blocking something in the Boot Camp installer.
Turning off MaCafee antivirus allowed the update to install correctly.
It's something so simple, but can cost a LOT of time.