Do you know about the unofficial "backward saddle" code?
Frequent vélibbers turn the saddle backwards on faulty bikes, so that others don’t take them by mistake.
If you find a bike with the saddle pointing backwards, don’t just turn it round again: check it out! It’s probably broken in some way.
If you borrow a bike and it turns out to be faulty, spin the saddle round when you return it (by releasing the same lever that you use to set the height) to warn others. (If - as is often the case - I have a block of Post-Its with me, I’ll stick one on the handlebars describing less obvious (…)