1. Play Minecraft.
2. Press mouse 4 or mouse 5 in windowed mode and it opens a menu (unwanted behaviour not intended or specified by any possible control binding within the game).
=== Background & specifics
I'm using mouse 4 (side-back button) as Push-To-Talk in TeamSpeak while playing Minecraft.
My mouse is Razer, and Razer config is bound to use mouse 4 as "Back" button, and mouse 5 (side-front button) bound as "Forward" button (as used in browsers).
When playing MC 1.4.2, 1.4.4, 1.4.5 (and previous public version 1.3.x), or apparently any "old launcher" version up to and including 1.5.2, in windowed mode (enlarged to fit screen) and pressing mouse 4 "Back" or mouse 5 "Forward", Minecraft opens menu as if pressing ESC.