It can happen though, that a component can break due to wear or something can go wrong and unfortunately the user can fall and bruise. But when we design prosthetic components, we design them so that they can withstand a lot of situations so hopefully, not a lot of people falls.
Some amputees, especially the ones that have lost both legs, are less stable, as they have less muscles to control their body, so it is easier for them to fall.
Fortunately, the great team of physios and clinicians teaches all the amputees how to stand up and be more stable, as they are a lot of techniques that can help.
Preventing falls is one of the biggest challenges for people that design prosthetics, so we are always thinking about ways to prevent them, such as with the use of electronics like the ones that can be found on smart-phones (gyros and accelerometers).
I guess it will depend when the leg was faulty (like while crossing the street)! But having said this (and being dramatic at it), it is VERY unlikely for an artificial leg to fail, as they have safe mechanisms to avoid any risks associated with a possible failure, plus, as they have different electrical components, they need to be maintained every so often, sometimes they even require a software upgrade, like computers and mobiles!