Explain the UniquedById example in the manual

New to Julia
1

The manual section Module initialization and precompilation gives this example of a bad pattern:

mutable struct UniquedById
    myid::Int
    let counter = 0
        UniquedById() = new(counter += 1)
    end
end

All subsequent usages of this incrementally compiled module will start from that same counter value.

I am trying to recreate the full scenario where this code becomes an issue. I can put this code in a package and use it in a script, the package will be precompiled and cached. What would be my next steps that will reveal the issue?