Agreed, 1/2 cup won't bother you at all. I did something similar on my first batch. I was holding my better bottle, and set it down so that the the blow-off tube landed in the water. Holding the bottle compressed it, as I let go the sides expanded and sucked the water right in.
This is why I really like the three piece airlocks, you can't suck liquid backwards (as long as you don't overfill).