SCCM has a set of reserved site codes that Microsoft does not recommend you use when building your SCCM environment.
Thanks to Torsten Meringer @tmeringer for pointing out that I’d used one of them in my R2 build environment! So time to update here.
MS KB article on the subject
http://support.microsoft.com/kb/279868/en-us
Note if you assign a site code you cannot change this. Time to uninstall and reinstall SCCM
SCCMentor - Paul Winstanley 
I would say that in mentioned KB article is one important reserved name missing “SMS”. Several our customers used it for SCCM 2012R2 hierarchy (until SCCM 2012SP2/R2 SP1 it wasn’t big deal, but after upgrade it’s a BIG problem. I had to modify SQL stored procedure for generating collection names, because out of box collections have SMS prefix reserved). Now for SCCM 2012 and SCCM current branch (1511) they updated reserved names in supported configurations (section “Specify a Site Code and Site Name”).Here are links:https://technet.microsoft.com/en-us/library/gg712320.aspx#BKMK_SiteNaming
https://technet.microsoft.com/en-us/library/mt590813.aspx
MS should update also names inside KB279868.
Kind Regards,
Martin
Thanks Martin. Makes perfect sense. I’ll update my ConfigMgr step by step docs that link to the KB.
Martin – I’ve emailed MS to get them to update the old KB to reflect this additional reserved site code.
Martin – smsdocs have updated the article to reflect the addition of SMS