The slip-up I found is in one of the last scenes, when Lenny and Evan burn Evan's stuff.

First of all something important to know, to understand why (I think) there is a Slip-up.:

If you have seen the movie, you should know that Evan got his blackouts because of himself, returning to the same point later. You should also know that he started the journals because of his blackouts.

So now to the Slip-up.

In the ending, he hides under a desk and watches the movie his mother made in his childhood, where he meets Kayleigh the first time. When he "goes beck" and says something like "I never want to see you again or I kill you and your family, we get to know that they never see each other again(just forget about the last scene for a moment), so they never were togeter in the Millers house, blowing up the mailbox or fighting on the junkyard.

It get's a bit complicated now, but it's reasonable, if you think about it.

After that, Evan never had to change his life. He never had to go back , so he never actually got Blackouts. Because the blackouts were caused by him going back, and he never had to go back, he never got any blackouts further in his life.

SO: He never had a reason to start a Journal.

And NOW to the real slip-up:

In one of the last scenes, you see Evan and Lenny burning things from his past. There are films and photograps, BUT there are also jornals. Where do they come from? He never had to start them.

It's a bit complicated, as I mentioned earlier, but if you think about it, it's true.