There are many things that people believe to be true, but actually aren't. This is definitely true about pet care and veterinary medicine. So, inspired by this list on Wikipedia, I'm going to do occasional themes based on common misconceptions about pets and vets.
Neutering will calm down my dog/cat.
Spaying and neutering only affects behaviors related to gender-based behaviors. This can include territoriality, aggression, sexual desire, and so on. However, it does not include activity levels. A pet that is excited and hyperactive before neutering will be the same after surgery. Being sexually intact is irrelevant to such behaviors.
Neutering will make my male dog/cat lazy.
As with the first misconception, spaying and neutering has no affect on most pet behaviors. Removing the gonads (sexual organs producing hormones) has nothing to do with a pet's energy levels or overall behavior and attitudes.
If I come home and discover my dog urinated/defecated on the floor, rubbing his nose in it will teach him not to do it again.
Studies have shown that unless a punishment or reward is given within 20 seconds of the behavior, the dog will not associate the reinforcement (positive or negative) with the behavior they performed. This means that punishing a pet more than a few seconds after the behavior has no effect at all, other than making them scared of you.
When my dog does something bad, he knows it and acts guilty.
Have you ever come home to a knocked-over trash can, and your dog runs and hides with their head down and tail tucked? We perceive that as guilt, and they know that they did something wrong. Well, that's not how canine behavior works. Remember, punishment must be given with few seconds to be effective. Try the following some time...when your dog isn't in the room, turn over the trash can, then walk away. Wait until your dog goes into the room and sees the mess, then see how they act. Their behavior will be the same as if they had gotten into the trash themselves. In a dog's mind, they associate there being a mess on the floor with getting punishment from their owner. They don't make the connection that you're mad that they made the mess. They make the connection between the presence of a mess and your reaction.
I'll post more as they come to mind!