Real examples "We've never done that." or "That's never a problem for us.", "In my entire career I've never done it that way". At which point there are almost always examples, but really all its doing is shutting down the conversation.

These statements generally come from an emotional reaction (or gut-feel) which are fine since we are humans after all... but really do nothing to help have a rational conversation and make a smart decision.

Wishy-washiness is certainly another problem, but that's a whole different topic.