Incorrect upgrade sql logic
Now when you write upgrade sql instructions in install.xml the system works a bit incorrect.
let's imagine you changed one field for personal blog table, you make some intermediate file changes
Your personal blog version released with the core was 3.1.0 but when you release a new core version, it might require one db change. I don't know what version is used now on a customer website BUT I do know I must run update for any versions from 3.1.0. We need to implement a way to run queries for the version you install now, not for a previous one.