Modern F1 rules heavily restrict testing. During the season only straight line aero runs and tests with 2-year old cars and demotires are allowed. Yesterday the whole F1 paddock found out that Pirelli and Mercedes were testing for 3 days (or 1000km) after the Barcelona race. This is more than big news: Mercedes now got important data about the tires which will be used during the rest of the 2013 season, and Pirelli even brought 2014 prototype tires along.
Every other team is "not amused". Red Bull is planing on filing a offical protest, as they think that Mercedes broke the "no testing" rules.
Pirelli says that this was all legal as they have the right to hold a 1000km test during the season to react to problems and to prepare for the next season. They might have a F1 car of their own, but it is basically a 3 year old Renault which laptimes are 5 seconds off pace. They claim that the only way the could really test the tires was a current car, and so Mercedes was invited.
To make matters worse both Mercedes and Pirelli decided to keep the test a secret. Why would you keep something legal secret?
It will be interesting to see how this story turns out.