I made a pretty big starter the other day. I made it much larger than normal because it was the first time I was using washed yeast so I put a lot of yeast and wort in there. The krausen on the starter did get pretty high and I was a bit worried about blowover, but it didn't happen. This was at about 70 degrees w/ 1056 yeast... In hindsight I probably used way too much yeast and way too much extract in the wort so under "normal" circumstances I don't expect it to happen again.
I just made up a starter tonight and I couldn't find the bung for the airlock on my 1/2 gallon jug that I use for the starter. So I just covered the just with sanitized foil... Wort volume is 1 quart, so there's quite a bit of head space. Is this going to be a problem?
A few months back I made a starter with some Wyeast 1275 I'd harvested from a Stout. I had about 3 cups of wort (1/2 cup of DME) in a 1 liter mason jar with tinfoil over the top. Not sure of the fermentation temp. When I got up the next morning the tinfoil was lying on the counter beside the jar and krausen was still flowing over the top of the jar. I think the krausen must have formed a seal with the foil and then the CO2 popped the foil right off the jar. Pitched it anyway (into a batch of pumpkin ale) and it turned out great - no infection. I did use a blowoff in the primary.