I kind of got a theory going about the rest of the stuff in Triangle, but this really escapes me. Why did Jess have to write on the mirror again? Did someone wipe off the previous message? Who? Why? It's not like a lot of time had passed.
This movie shows the endless looping, which started when she killed Tommy and herself in a car accident.
Here I am using numbers to denote the revolution.
In the first revolution, Jess-1 sees the message Go to Theater.
In the second revolution, Jess-2 recognizes the hallway and the on her way to the dining room, she sees the message was already there. She wipes that message off.
In the third revolution, Jess-3 writes that message. She tries to get out of the loop this time.
So, this message was written by Jess-3, and seen by Jess-1 and Jess-2. But Jess-2 wiped this message off when Jess-1 left. This is what we saw in the movie. However, this wasn't the first loop when she and her friends got on the boat. By looking at the number of dead bodies, it can be around 50-60th.
Since this is the endless loop, we can assume that she wrote and erased that message in the same way as mentioned before.