It's not clear to me whether the behavior described below is a bug or a design decision (possibly for efficiency). It is a divergence from CPython behavior. (My apologies if this has been discussed elsewhere - a few quick searches did not turn up anything for me.)

Consider the following (sloppy) code:

def outer():
    x = 1
    def inner():
        x += 1
        print(x)
    inner()

On CPython calling this function triggers an 'UnboundLocalError':

>>> outer()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in outer
  File "<stdin>", line 4, in inner
UnboundLocalError: local variable 'x' referenced before assignment

In MicroPython/CircuitPython calling this function prints a value:

>>> outer()
2

Micropython does not have an UnboundLocalError exception... so perhaps a NameError could be raised instead?...

... or the 'MicroPython differences from CPython' documentation could be updated to add this difference (if it is, in fact, a design decision for efficiency).