Input file with a subtle error: a number where an assignment target is required:

for 1 in []: pass

Run it, it gives a SyntaxError.  Note how the caret pointing to the incorrect token is position one to the left of where you'd expect it:

  File "s.py", line 1
    for 1 in []: pass
       ^
SyntaxError: can't assign to literal

For every syntax error I've seen that's produced by ast.c this seems to be the case -- the caret is always positioned 1 too far left.

I tried to understand how this is happening but my AST-fu is lacking. It seems this has been happening since ast.c started added column numbers -- in Python 2.7 there's no caret at all, but in 3.4 and later there's a caret and it has the same problem. (Also in 3.3; I don't have 3.2 or older 3.x lying around to test.)