The original license for the WebM project had a clause that if you brought patent action against Google, the patent license was terminated. This isn't unusual in open source licenses (the second Apache license and version 3 of the GPL have something similar), but the WebM license had an additional problem.
"The twist was that ours terminated 'any' rights and not just rights to the patents, which made our license GPLv3 and GPLv2 incompatible," explains Google's Chris DiBona, "Also, in doing this, we effectively created a potentially new open source copyright license, something we are loath to do."
They've resolved the issue by decoupling patents from copyright, meaning the copyright part is now a pure BSD license. They've used "patent language borrowed from both the Apache and GPLv3 patent clauses" for their own patent clause. They're no longer creating a new license, and the patent clause can stand on its own. Additionally, they have updated the patent grant language to make it clear that it includes the right to modify the code and give it away to others.
"Thanks for your patience as we worked through this, and we hope you like, enjoy (and most importantly) use WebM and join with us in creating more freedom online," DiBiona adds, "We had a lot of help on these changes, so thanks to our friends in open source and free software who traded many emails, often at odd hours, with us."
A good solution from Google.