One of the many notable parts of the first teaser for Star Wars: The Last Jedi was the brief appearance of Kylo Ren. He only appeared in one shot, standing his ground with his crossguard lightsaber ignited and ready for battle, but it was his face that really attracted the attention.
When we last saw him, he'd been defeated in lightsaber combat with Rey, who'd left him with a pretty nasty looking scar running down his face. However, when we see him in this trailer, the scar looks noticeably different - it's smaller, and has shifted across slightly. You can see in the image above what it looked like in The Force Awakens, and how it looks in the trailer for The Last Jedi.
It's not a major change, but it was something a lot of fans picked up on. Because of that, director Rian Johnson was asked about it on Twitter, and he has actually came out and issued a response. Initially he did say that it hadn't moved at all, though it does appear as though he was just joking at first:
That's fair enough, really. It doesn't exactly make any difference to the character, and if it looks better this way then so be it.