Repeatable proxy 502 errors during apt-get update, e.g.:

W: Failed to fetch 
http://ppa.quickbuild.pearsoncomputing.net/trinity/trinity-r14.0.0/ubuntu/dists/jessie/main/binary-i386/Packages  
502  Proxy Error

Appears to be due to no DNS A record for:
  ppa2.quickbuild.raptorengineering.com

FWIW there is an A record for the above if "2" omitted.

--Mike