Needs full stack trace, but this type of issue is typically caused by multiple threads manipulating the same iterable at the same time.
[2021-01-23 19:03:45,694] sopel.bot ERROR - Unexpected error (dictionary changed size during iteration) from Voidwalker at 2021-01-23 19:03:45.694143. Message was: VoidTraceback (most recent call last): File "/srv/sopelbots/devvenv/lib/python3.7/site-packages/sopel/bot.py", line 606, in call exit_code = func(sopel, trigger) File "/srv/sopelbots/devvenv/lib/python3.7/site-packages/MirahezeBots/plugins/welcome.py", Jan 23 19:03:45 100 sopel[24937]: [2021-01-23 19:03:45,694] sopel.bot ERROR - Unexpected error (dictionary changed size during iteration) from Voidwalker at 2021-01-23 19:03:45.694143. Message was: Void Jan 23 19:03:45 100 sopel[24937]: Traceback (most recent call last): Jan 23 19:03:45 100 sopel[24937]: File "/srv/sopelbots/devvenv/lib/python3.7/site-packages/sopel/bot.py", line 606, in call Jan 23 19:03:45 100 sopel[24937]: exit_code = func(sopel, trigger) Jan 23 19:03:45 100 sopel[24937]: File "/srv/sopelbots/devvenv/lib/python3.7/site-packages/MirahezeBots/plugins/welcome.py", line 80, in welcome_user Jan 23 19:03:45 100 sopel[24937]: save_known_users_list(bot.known_users_filename, bot.known_users_list) Jan 23 19:03:45 100 sopel[24937]: File "/srv/sopelbots/devvenv/lib/python3.7/site-packages/MirahezeBots/plugins/welcome.py", line 52, in save_known_users_list Jan 23 19:03:45 100 sopel[24937]: for channel in known_users_list: Jan 23 19:03:45 100 sopel[24937]: RuntimeError: dictionary changed size during iteration