Umpires called a balk against Max Scherzer on Friday when he appeared to pitch too fast:

Scherzer tested the boundaries of baseball’s novel pace-of-play rules during his second spring training start, trying several unusual tactics to get Washington Nationals hitters off their game. At one point, he started throwing a pitch to Victor Robles the moment plate umpire Jeremy Riggs reset the clock, and Riggs called Scherzer for a balk.

“He calls time, I come set, I get the green light,” Scherzer said. “I thought that was a clean pitch. He said no. We have to figure out where the limit is.”

ESPN.com

So why was this a balk? There are two things in this article about the set position that may be involved. The pitcher must come to a complete stop in the set position before delivering the ball. The interpretation might be that the stop doesn’t start until the umpire signals play. Scherzer started his motion immediately. There is also a prohibition on quick pitching:

With no runners on base, the pitcher is not required to come to a complete stop when using the Set Position.  If, however, in the umpire’s judgment, a pitcher delivers the ball in a deliberate effort to catch the batter off guard, this delivery shall be deemed a quick pitch, for which the penalty is a ball.  See Rule 6.02(a)(5) Comment (Rule 8.05(e) Comment).

BaseballRulesAcademy.com

Scherzer worked on throwing off batters by slowing down and speeding up his time between pitches. I suspect few will do this, as most pitcher likely want to get a rhythm going.