Here is an image from the same project opened in 4.1 and 4.2
Preferred behavior is to see animation fade out. We can ignore this issue since it fixes itself in 4.2 However I am failing to set an interpolation for alpha at the end of the animation in 4.1
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I'm not understanding the problem! 
What do you see? What do expect instead?
False! These are not the same picture. The difference is clear to anyone with functioning eyes. Observe:
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Aha, now I understand. Hard to say unless you showed how you created such keys. My guess would be one key is a tiny bit past the other, like 0.00001. I'm glad it's fixed in 4.2 though! No need to worry unless you see it in 4.2.
