The first three drives of Ohio State vs. Michigan ended in punts. The fourth drive was set to end in a punt, but on a fourth-and-7 from the 9-yard line, Tyree Kinnell was called for roughing the kicker:
That gave Ohio State a first down and they made the most of it. Ezekiel Elliott ran for 66 yards and a few plays later, J.T. Barrett got a touchdown for the game's first score.
Jim Harbaugh didn't like the call, as cameras caught him laugh-yelling on the sideline. Surprising, I know.
The "roughing" call was a big deal. If the call was just "running into the punter," Ohio State would've faced fourth-and-2. But the call was "roughing," which is an automatic first down. The rulebook specifies that a roughing call "endangers" the kicker and doesn't merely "displace" him. Generally, that's interpreted as anything involving the plant leg, which this one didn't.
The other reason Harbaugh would've had to be upset is that Ohio State's punter, Cameron Johnston, was taking a running start towards a rugby-style directional punt to the sideline. It's hard to ensure you don't hit a punter after that running start. But until the punter leaves the tackle box, he is protected by these rules.