Pip installs to unexpected place

[Grant Edwards]

On 2025-04-15, Thomas Passin via Python-list <python-list at python.org> wrote:

> On Linux, at least, it's standard for pip to install into the user's 
> site-packages location if it's not invoked with admin privileges - even 
> without --user. Pip will emit a message saying so. Well, that used to be 
> true but nowadays Pip wants you to use the --break-system-packages flag 
> if you want to insist on installing into the system's Python install, 
> even if it's going to go into --user.

I've always been a little baffled by that message when installing with
--user.  How can that possibly break system stuff?