Real example (this actually happened to me):
Opponent has a Treetop Village and Scavenging Ooze. I am at 2 life with Cryptic Command in hand. He offers to move to the beginning of combat. I say okay. He swings with his Scavenging Ooze. I say "Whoah, hang on a second. We are in the beginning of combat." He calls a judge and the judge agrees. I can tell that his actions were calculated, and so for the rest of that game I am playing competitively (or as you call it, "unsportsmanlike").