Yeah, if an Engineer isn't paying attention they can really get fumbled. I hate it when people can't play Engineer worth ****; I like to call them "derpgineers". We all know one. The kind that when you ask them to repair your strategically placed level 3 sentry will instead use their precious metal on building a level 1 right near the front lines.
All class teams almost never work though, even Engineer on defense, because of that reason... but if someone was smart enough to place a sentry in that area and monitor it, I don't think that would have happened.