Sniper rush

From TF2 Wiki

The Sniper rush is very rare. Unless your whole team can aim extremely well, it will likely fail. If your team is made up of effective players, then a Sniper rushes biggest impediment is fast classes, Sentry guns and Spies. The Sniper rush is most effective on maps which mantain large stretches of open space. Maps with tight choke points or an abundance of corridors can cripple the Sniper rush.