Sure, anyone can be wrong. But if some claims to be a prophet and their fruit stinks we can steer clear of them. If someone claims the authority of an OT prophet they need to live up to the standards of an OT prophet, which means one false move and they lose their title (and they should be thankful they don't lose their life, but we don't stone people these days regardless of what they have done).
In the NT approach it's clear that we should test prophecies. As to how many test it or how rigorously they test it, that would depend on the impact that following it would have. If following the word would have minimal cost associated with it then there's probably no need to test it too thoroughly, simply because if it were totally made up the impact would be minimal. If it would have major repercussions then it should be tested and tested and tested again. The God who was capable of sending the message through one channel is quite capable of confirming it through another if that's what is required.
