One of the things I've always found interesting in the Zimbabwe series is that the first notes in the 3rd dollar series (from $1 to $1000) were released on Aug 1 2008 and the 4th dollar notes (from $1 to $500) were released just 5 months later in Jan 2009. But 1 4th dollar was worth 1 TRILLION 3rd dollars. So they couldn't have people confusing the two note series, which were circulating very much at the same time. It would not have been good if there was even a reasonable chance that someone might think one of the "old" 3rd dollar notes was a 4th dollar note. So, when you look at examples of each for the different denominations, you can see how hard they were trying to make sure there was not confusion.
I think the most striking thing for me is the coloring and the changes to where the denominations are, but, if you're paying attention at all, you're not going to mistake one for the other.