Manual

[Ulrich Neumerkel]

Node: Keys

In this section C-g should be mentioned and the fact that the prefix
is prompted in the echo line.

Similar to C-h, C-g

o can be applied in almost all situations (to abort prefixes and
  minibuffer)

o is not shown in the keybinings.

This seems to be a frequent problem with beginners using EMACS:

a) You have to look at the echo line to see that a prefix has already
   been typed

b) you have to know C-g to abort (most?) commands safely