In the current dev build (v0.200.219) a ship with 100.0% reactor usage via chamber will behave as if it's overloaded.
Specifically, i noticed on a small ship i'm using to test stealth. The chambers give permanent stealth, stealth level 4, jamming, and cloaking. This was working a few days ago when i last used the ship, and dropping stealth level down to 3 (90% reactor usage) reactivates the chambers. Bumping back up to stealth level 4 pushes me back to 100.0% and overloads the reactor.
I really should bookmark the bug tracker, since i'm probably not accomplishing anything but letting forumers know. The devs are probably already on it.