In our work, we encounter issues in core or contrib that trip us up.
Let's add those to a list here, and host sessions to learn drupal dev through mentored contribution.
Tasks can be tagged by difficulty level and skills needed.
A mentor could host a session where they pick an issue to work on and live code their way through with the group's input.
Another session could be going through the list of issues, discussing how to move forward, and assigning to mentees to work on.