The Store button on the main menu has animated icons attached to it. When you do anything that prevents those animations from being displayed, and then return to the menu, all the animation updates that would have taken place during that time are played at high speed. This includes:

• Opening the worlds list (clicking the Play button),
• Opening the Achievements UI, Settings UI, or Store UI,
• Opening the skin picker UI, and
• Minimizing the window
  (I suspect that sending the app to the background would do this as well, but I'm not able to test that.)

Steps to reproduce:

1. Start Minecraft.
2. Note the normal speed of the animations on the Store button.
3. Click any menu button or the skin picker button, or minimize the window.
4. Wait 10 seconds.
5. Return to the main menu or restore the window.

Expected results:
The animation on the Store button runs at normal speed.

Actual results:
The animation runs at high speed for a period of time proportional to how long you were away from the menu, then resumes running at normal speed.