This fun little test is I would guess the adult or modern equivalent of the spot the difference puzzles I used to love when I was growing up. Like everything once you know where to look you will be angry with yourself for not spotting the various strange things in the first place.
I found as I went through the test, my eyes and brain became more accustomed at what to look for, and I did progressively better, but I still enjoyed the challenge. For all you eagle-eyed people out there who enjoy a visual challenge, this is a fun little test. Let us know how you get on in the comments.