Size and Weight:
When the skull of Nanotyrannus ("tiny tyrant") was discovered in 1942, it was identified as belonging to another dinosaur, Albertosaurus--but upon closer study, researchers speculated that it might have been left by an entirely new genus of tyrannosaur. Today, opinion is divided into two camps: some paleontologists believe Nanotyrannus indeed deserves its own genus, while others insist it's a juvenile Tyrannosaurus Rex.
Further complicating matters, it's possible that Nanotyrannus wasn't a tyrannosaur at all, but a dromaeosaur (the class of small, carnivorous, bipedal dinosaurs better known as raptors). Until more fossils are found--and until now, the evidence has been scanty at best--the arguments will likely continue.