Minecraft failing to recognise release of key when less memory is allocated

What I expected to happen was...:
To stop moving/ascending/descending when I release W/A/S/D/Space/Shift

What actually happened was...:
Sometimes the player keeps moving/ascending/descending even when the key is released.

Steps to Reproduce:
1.Allocate 200MB to Minecraft.
2.Start a world.
3.Keep moving until the bug occurs.