A single supernova has been found that does not follow what we know about supernovae. What's known as Type Ia supernovae start off as white dwarfs, the remnants of stars like our own, with more massive companions. The buddy dumps extra mass onto the WD, pushing it up to exactly 1.4 solar masses (according to the old Chandresekhar theory) at which point it goes *BOOM!*
Since all of these started off as the same size, they make the same size boom, or in astronomer's terms their light curve is the same (with minor corrections which aren't yet understood but can be accounted for). Because of this handy-dandy little fact that they all look the same really, we can use how bright they appear to us here on Earth to determine how far they really were. This is one small piece of the puzzle that tells us the Universe is 13.7 billion years old, is accelerating its expansion, is geometrically flat, infinite in size, and contains 70% dark energy.
This group has found evidence that the supernova known as SNLS-03D3bb was bigger than 1.4 solar masses (2 solar masses was reported elsewhere) when it exploded.
If the WD wasn't 1.4 solar masses to start, we don't know how big its *BOOM!* was, we don't know how bright it really was, we don't know how far away it really was. If this was just one star, no biggie, it's a freak. If this were MANY stars, we don't know how bright any of them really were, and therefore we don't know the universe's age, acceleration, shape, size, or composition.
This one anomaly isn't going to send the whole theory of the universe down the drain, but it is disturbing. Believe you me that there will be a LOT of double checking of work in the next year or two, you'll probably hear about some in early January at the next big national (and international) astronomy conference, and after that if it's not just an anomaly textbooks will be rewritten again.
(And they'll get the fix that pesky Pluto typo too.)