When driving a manual car, you have to use a lot of clutch control in stop, start traffic. Pressing the clutch to slow down, raising the clutch to move more and then dipping the clutch back down when stopping. While doing this you might find that your left foot slips up the clutch pedal. Sometimes it might slip up so high that you have to do some kind of wiggle to get your foot back down the clutch pedal.
The reason why your foot slips up the clutch pedal is probably because you rest the heel of your left foot on the floor as you raise the clutch up.
To avoid this problem, you could try hovering your heel off the floor as you press the clutch and once you raise the clutch near or at the biting point then rest your heel on the floor for more support for the final part of releasing the clutch.