When pressing Alt key to temporarily turn off snapping, it could be turning off also curve closing.

 From:  Michael Gibson
5338.6 In reply to 5338.5 
Hi Brian, yup usually it's good to leave both Object Snap and Straight Snap on most of the time.

But sometimes when you're drawing curves you might find that you can't place a point quite in a location that you might want for shaping the curve because it's snapping to other points in that general neighborhood. If that happens then holding down Alt is a quick remedy for that, since it only disables the snapping while the key is held down and will return to regular operation once you release the Alt key.

- Michael