This is related to https://bugs.webkit.org/show_bug.cgi?id=26162 and may be the same cause. But the behavior is very different.
If you have accelerated compositing on and run the test case in the bug above, it transitions when you hover like it should. But when you unhover, the two transform animations continue to move in very odd directions.
Sorry, it is actually related to https://bugs.webkit.org/show_bug.cgi?id=26163.
Same root cause as bug 26163.
*** This bug has been marked as a duplicate of 26163 ***