Recently I had to install a new water heater, usually a simple but expensive process.  Not for me thou.  Back in the past when my old one was installed they reversed the flow of water from its normal course.  So when the new plumber came in, they set it up the normal way and turned it on.  It then proceeded to blow 10 years of muck out of my boiler and into my hot water stream.  What a mess!  It took a week to get the hot water to look clear again.