Ive been on teams where this has happend, and ive stopped it when they have tried to do it to us. It helps if your spy and you can easily stop it, if your ever on PLMaps they do it there.
And yes, if you come play with the late night crowd on PL, hobo and that crew, this happens a good amount. It still baffles me that they knew they were back there (that soldier saw the medic in the spawn). 1 good spy can clean that up, but it is so much fun to pull off.
I love being offensive engy, you can do really good with it. And oddly I am much better with the shotgun than many other weapons. Oh well, that was pretty nice!