Well I have had to learn the hard way. Now I take the time to try and understand what needs to be done in order to remove the nut without breaking the bolt.
I will try to explain the technique for removing the nut without breaking the bolt........
First, I spray the nut and bolt with some type of penetrating fluid such as"rusty" or wd-40 or something simular. Then I use a small propane torch and warm only the nut up that is on the bolt. Sometimes this works the first time and all is good. If not, I continue to use a small torch, not a big torch, and continue to heat the nut so it will start to expand away from the bolt. A word of caution, WEAR FACE AND HAND PROTECTION. sometimes applying rust penetrive will bubble and or pop and spit back at ya or can catch fire. be prepared with a squirt bottle of water. I continue to heat and apply penetrating fluid. You can actually see the fluid get sucked in between the nut and bolt. I then use a socket and or box wrench to slowly and rather gently try and turn the nut back and forth to break it free. Once broke free, i continue to loosen and then tighten the nut which allows the threads on the bolt and nut to kinda clean themselves from the corrosion. Time and patients usually pay off and the nut can be removed from the bolt with out breaking it.
This also works well on rear brake lever that have rusted from the bike sitting outside in the weather for years. Instead of heating the leaver, I heat the tubing that contains the shaft that the lever go into. Heat the tubing, apply penetrating fluid and begin to try and work it back and forth. Eventually it will break free.
I hope this helps! Remember, patients patients patients!