Not sure if bugs count, howerer there's an issue with type inference and existential types that has now been fixed (but the fix has not yet been released, so the issue is still there in the current release 2.10.0)
It's SI-5330. In the description of the issue (and in the various comments),there are examples of code that used to compile in scala 2.9 and is now failing to compile in 2.10.