Scheme Error Messages, Part 1

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Unbound or unassigned variable
Either a typographical error, or use of a variable outside of the part of the program in which it is available (e.g. using a variable created by let outside the body of the let, or use of a variable created by define within a procedure outside of that procedure).
Applying something that isn't a procedure
"The object ... is not applicable" The value of the first subexpression in a combination must be a procedure.
Wrong number of arguments to a procedure
The number of arguments in a call to a procedure must match the number of parameters the procedure expects.

Jim Miller W3C