To add on to what Tea Drinker says, which is what happens on contact with metal, I don't ever use a chainsaw without wearing goggles, a helmet with mesh visor, protective chaps, heavy-duty gloves (make sure they don't go up past your wrist, or they will be easy to catch onto), and tight-fitting heavy-duty long sleeve shirt. I've never had any accidents. I think safe is better than sorry, especially when working on clients' properties, to give them a feeling of security.
You will go dull within seconds of hitting steel. I haven't had a chain fly off for this reason, but it is possible, so always dress prepared, and don't let yourself get comfortable. It's very inefficient time and money wise to have to happen chainsaw blades every few minutes, so if you have a tree which is riddled with steel, it's best to leave it.
Also of note: in areas like where I am, where hunting is a big thing, it's common to find lead bullets buried in the tree, and they can be anywhere in the diameter, depending on how long ago the shot was fired. These will dull the blades, but not nearly as much as steel. Just note that lead poisoning is not fun, so watch that you don't inhale the sawdust or rub it into your skin.