Patch 5.2.0a is the unofficial name for the patch build 16669 that came out a few days after the official Patch 5.2.0 build 16650. Reliable info on this patch is scarce, but this client could correspond to the March 7, 2013 slate of hotfixes.
So far the only official information known is the acknowledgement that Blizzard "released a small client patch to resolve some bugs".
^Jurannok 2013-03-08. #4 - Downloading Patch Info several Times. Official Support > Technical Support forum (US). Kalgalath, We just released a small client patch to resolve some bugs. Today's patch is very small and updates the game to version 16669. Tuesday's release was version 16650.