Not answering the specification. How many times has a client received a proposal that doesn't really provide a solution to what was proposed?
Not including all the required information. How does the client make an informed decision?
The wrong name in the proposal. Who is the proposal for? Did the proposal just use a load of cut and paste from an earlier proposal?
Elementary and stupid mistakes maybe. But now ask yourself...have you ever done any of them? I bet you didn't win the work.