Okay, paranoia check. I have a 2L starter going on a stirplate. Got home and found that the aluminum foil cap had been lifted off with the flask opening very slightly visible. Should I be worried about contamination or while the stirplate action be blowing out the baddies?
Thanks
No worries. Unless your kitchen/basement/whatever is absolutely filthy, you could probably leave the foil off and not have any problems. Yeast are pretty good at killing off the baddies, and they’re going to have a helluva numerical advantage too.
+1 You should be fine.
Agreed. RDWHAHB
Unless you have these guys in the house. I probably won’t even brew till first freeze now.
A freind of mine has mold in her basement and asked me the same question. My response was to generate the starter using an airlock. I know that it is perfectly fine using aluminum foil covering the flask but it is better to be safe than to risk an infected starter.
On that note…I agree with the other posters in that assuming your home is fairly clean and mold free you should be AOK. 
Thanks all: brewed yesterday-the flask was filled with yeast (still on the stir plate). No mold in the house-very dry climate. Should be fine and thanks for the advice.