Luke sees a vision of Leia and Han being tortured and decides he has to go to Lando's mining colony. Yoda tries to stop him by saying "If you go now, help them you could, but you would destroy all for which they have fought and suffered."
Ummm... HOW, exactly, little green dude? Considering that all they have "fought and suffered" to do for the last hour or so was stay the heck out of Vader's grasp! It's not like Luke's arrival single-handedly led to the downfall of the rebellion or anything. In fact, Luke going to Cloud City seemed to not have any major benefit or detriment at all outside of Luke losing his hand (maybe they "fought and suffered" to make a cybernetic hand only to feel devestated when they had to give it to Luke?)
So really, what's with Yoda's warning?