This is a really sore spot with me. People in this business are, on average, horrible at testing.
They see a change and they assume "Hey, this change is because of the last thing I did." More than likely it was related to something someone else did, or to something they did so long ago that they have forgotten what they did.
Then, they fail to do reverse testing and they fail to try to replicate their test results on other sites.
These failures are caused by laziness and this costs these people huge amounts of wasted investment and lost revenue.
It makes me crazy to watch people do this over and over again. It's like watching a person stick their hand into a blender -- repetitively.