Sometimes when I crash cool, it does draw back in.
When fermenting lagers, I press on the pail lid to get a little negative pressure to get the water higher in the tube under the bubbler, to see if it is done fermenting. I usually use DW with a little shot of star san.
I always fill my airlocks with starsan just in case this does happen. A good tip when crash-cooling a beer is to use a single s-shaped airlock (not the 3 piece ones) and put just a small amount of starsan in it. That way when the beer cools and the pressure difference changes, the liquid can't be sucked back into the fermenter into your finished product. This will happen if you use the 3 piece airlocks though. I always learn my tips the hard way of course.