A global variable reportedly is not there (NameError: name 'a' isn't defined) although it should be there.
This occurs in a setting of a recursion test function that hits the limit of 19 recursions on the second thread/core on a RPi2040.
The global variable, a, is updated in this function.
Strangely this only occurs if also two other global variables are defined, sleep_ms is imported as a single import and the maximum recursion depth exception is catched with try/except.
The code is as follows:

import _thread
from time import sleep_ms #, sleep_us  # if more than one functions imported there is no NameError

a = 0
b = 0

def recursionTest():
    global a
    a += 1
    try:
        recursionTest()
    except Exception as errorMsg:
        print(errorMsg)

_thread.start_new_thread(recursionTest,())

c = 0
sleep_ms(10)
print('recursion depth:', a)

The output is as follows:

Unhandled exception in thread started by <function recursionTest at 0x20008d50>
Traceback (most recent call last):
  File "<stdin>", line 9, in recursionTest
NameError: name 'a' isn't defined
recursion depth: 0

If c = 0, for example is commented out the output changes to (correct);

maximum recursion depth exceeded
recursion depth: 19

This was on `MicroPython v1.19.1 on 2022-07-27; Raspberry Pi Pico with RP2040