Other than the terrible mistake that taught me that planning ahead is a good idea, you mean? Man, that comic was a mess. I wrote myself into a corner in chapter 2, the antagonist wandered off and was never seen again, and nothing ever got explained.
I scrapped the project halfway through chapter 4.
I think my worst mistakes on a page-to-page level have involved reading order and placement of dialogue. I've drawn pages that were practically unreadable because it was super-unclear in which order you were supposed to read the panels.
I comfort myself with the fact that I was at least never as bad as the Swedish translation of the manga-Bible (literally the Bible, the Books of Moses, in manga-format) which had all the panels arranged to be read right-to-left - but then put all the dialogue balloons in each panel left to right.
Which means that yes, you were supposed to read it in two different directions at once.
It was the most atrocious thing I've ever seen.