What mistakes have you noticed on the show?

There's also one behind Private's First Prize, you know the fish trophy. One time during the episode Mask of the Raccoon, it was a safe where Private hid his Butterscotch Lollipop. In Work Order, it was a tunnel that led to the outside. :D

One of the things i've noticed from the begining was that they completely changed Malrene's eye color. In the first few episodes they were green, but now they're like hazel? Even on the box for the video game they're green! IT'S SO CREEPY!!! XD

Julien's plastic volcano changes a few times. Like in "Jungle Law," there was no hole; Julien was able to stand on it. Then, in "Huffin and Puffin," the volcano was pretty deep.

Also, in "Tagged," in that explosion, they came out of the volcano. How did they get to that room and into the volcano?

This has been asked before, and I noticed some mistakes too.

For example, in "All Choked up" Kowalski would need 700 years to make a shrinking ray. But in the episode "Jiggles" he did one when he was on the Kinder. >:(

On the episode "Mask of the Racoon" Archie knew the name of Skipper already...but no one ever told him, what Skipper's name was...x3

Marlene doesn't turn wild each time she goes out of the zoo :O

On the episode "Operation: Plush and Cover" when everybody was at the Zoovenirs shop, there were all the Mort plushies in a shelving.....under a sign that said "Mort".....do the Zoo keepers know the name of the animals??

XD

that kowalskis invention was taken by ronda the spy in roomies but in the epp huffin and puffin it was right behind hans...weird huh...

In "Huffin and Puffin," when Skipper turned up the heat in their lair, the pipes shown were in the sewer.
In "Whispers and Coups," when Skipper said, "...basic training will kick the love right out of you." Skipper was on the other side of the room than before.
In the beginning of the title theme song thing, the penguins are on the clock tower and jump onto Marlene's habitat and the otter habitat is not in front of the clock tower.
I noticed a lot more, but I can't think of anything else right now. :)

The clock in their lair... it always shows 12 o'clock... that's one thing. The other one is:
In "It's about time", when the time machine went crazy, the clock flew from the wall and Kowalski could just avoid it. But in the next scene, when he was about to jump in the portal, the clock was hanging right on the wall again... showing 12 o'clock...
I guess, it's.. when they animate it, they always have a background. The clock is part of that background and when something happens to it, they have to adapt it to the background - but they forgot it.

I realized that in early episodes, Marlene's fur is bright. Then later, her fur was darker...

in over phil when hes stuck in the trash first only his arm is sticking out and later hes in it just to his clerical:D

In Jungle Law, when Burt is chasing Julien, the sky moves. o.o