If your pet has a sudden change in behavior, make sure you get him/her to the vet as soon as possible. This may be a sign of something such as diabetes to something like a brain tumor or rabies. Just don't ignore it.
What specific changes should we be on the lookout for? I don't think I've experienced any sudden changes that did not seem warranted. I do believe that many people are likely to overlook certain changes though so this is good advice.
Changes In Behavior 
Linear Mode
