Reading so many manuscripts, I often see this thing that while I have never been formerly told was an error, just knew that it was. I was asked by several authors if it's okay to put to spaces after a period before starting a new sentence and I always tell them not to do it and I always fix it when I see it. I didn't know why, other than it just didn't look right.
While it's not one of my biggest petpeeves, it does cost me some time and money (when I have to hire someone else to fix it).
Here's one writer who hates "two-spacers," as he calls them. He not only angrily talks about his displeasure with the offense, he also gives references for why it's a no-no.