A continuity error is defined as any time in a piece of fiction where an event in the plot (whether small or large) contradicts information previously received. It has been stated that Happy Tree Friends relies largely on an absence on plot continuity to function. This allows them to kill any of the characters and still have them available to use in later episodes. However, in addition to characters coming back from the dead and/or miraculously losing major injuries, there are many other cases of continuity errors from episode to episode and even some within the same.
In the HTF BreakSeize the Day, Cuddles is clearly shown to suffer from photosensitive epilepsy. However, in In a Jam, he is completely unaffected by the flashing lights, even though they give Handy a seizure.
It is possible that Cuddles did not suffer a seizure because he was not focused on the lights, or due to the fact that he only had one eye, as obscuring one eye can lower the risk of seizures.) He was also seen amidst strobe lights in Keepin' it Reel and did not have a seizure (though these might have not been the correct frequency for seizures).
Toothy is presumably allergic to bees, as he gets stung by one in the eye in The Carpal Tunnel of Love, which results in it getting swollen. However, in Aw, Shucks! he encounters with a swarm of bees which sting him numerous times, but Toothy's body does not even swell up a little, which contradicts his "allergy".
There are two possible explanations to this last fact: The theory about Toothy's allergy is just a rumor from the show's audience, or the bugs that stung him in Aw, Shucks! are just wasps instead of bees.
Not sure if this counts as an "allergic reaction" or if it's just swelling.
The size of Lumpy's brain changes many times throughout the entire series. Lumpy's brain is suppose to be small enough to fit inside his small head, but there are some instances of Lumpy having a large brain, like in In a Jam and All Flocked Up.
In every almost episode, Lumpy's teeth are shown either on his nose or in his mouth. In You're Kraken Me Up however, his teeth are inside his nose, despite how every time the inside of nose is seen there's no teeth.
In Snow What? That's What!, Lumpy is shown relaxing shirtless in a hammock without being affected in any way, but in Just Desert, he is dressed in very heavy winter clothing to go hiking in the mountains. Furthermore, in Snow Place to Go, he is shivering in the cold along with everyone else.
Though this is likely because moose do not live as far north as the Arctic, which is why Lumpy chose to wear winter clothing in the former and why he was shivering in the latter.
In Wingin' It, Lumpy falls out of an airplane and, upon landing, has his leg bones push out his shoulders, but survives for the time being. However, in All Flocked Up, he falls out of a tree and splatters upon landing.
Lumpy's wealth throughout the series varies. In most episodes, he is shown to live in a trashy trailer and most likely unemployed in the majority of those episodes, but in several episodes such as Aw Shucks! and Peas in a Pod, he seems to have a very successful life and even owning a large house and having high paying jobs.
An example of Lumpy's large brain.
An example of Lumpy's tiny brain.
Proof that Lumpy has no teeth in his snout.
Now they are.
Lumpy loving the cold.
Lumpy miserable in it.
Moose don't climb trees, silly.
Hello? I just said they don't climb trees.
Lumpy's blue eye.
Lumpy with green eyes.
Lumpy with hooves.
Now Lumpy has toes.
Lumpy after falling out of a plane.
Lumpy after falling out of a tree. Also, an example of Lumpy's large brain.
Handy has his ears visible in Blind Date when his hat went up for a brief second. However, the same happened in Pet Peeve, but it seems he hasn't ears at all which is very odd since he is a beaver (like Toothy, who does indeed have visible ears).
In From A to Zoo, Flaky is clearly shown to be afraid of chicks, but in Take a Hike, she saves an eaglet without any sign of fear.
It is possible, however, that she is only afraid of certain kinds of birds.
Similarly, in both Ski Ya, Wouldn't Wanna Be Ya! and Mime to Five, Flaky is very clearly shown to suffer from acrophobia (fear of heights). However, once again in Take a Hike, she is not afraid to climb the tree to return the eaglet to its nest, and in Ski Patrol, she seems comfortable and happy on a ski-lift.
However, in Mime to Five, it could have been performance anxiety.
Flaky's conception on Flippy varies throughout the series. Flaky is also shown to be relatively calm about Flippy as she is seen giving him a cake knife, most likely aware that he has a tendency to flip out in Party Animal. However in Without a Hitch, she immediately freaks out about Flippy pulling out a knife to save her.
Flaky afraid of a chick.
Flaky not afraid of an eaglet.
Flaky's afraid of heights.
Flaky's as comfortable as Flaky gets with heights.
In Stealing the Spotlight, it is implied that The Mole cannot feel pain, as he does not notice that his thumb is on fire. However, in Double Whammy Part II, he notices when he cuts his foot on a piece of glass. (It is possible that he can only feel pain in certain parts of his body.)
His hearing in episodes can vary from good, to average, to almost completely deaf.