All breakers are tested and rated at 100% of their current, in free air at a maximum ambient of 40C (104F). The so called "80%" rule has to do with how they are USED. Circuit breakers protect conductors. For continuous loads, conductors must be sized to 125% of the max. load, so that means the conductors are never going to see more than 80% of the conductor rating (80% is the inverse of 125%). Since the CB is there to protect those conductors, it is therefore going to be loaded to more than 80% of its rating continuously. BECAUSE of that, circuit breaker PANELS are designed based on the breakers never seeing more than 80% of their rating continuously, so the fact that they are packed in there side by side and unable to have full air flow across all surfaces to dissipate heat is basically allowed based on that fact, and INDIRECTLY, you end up with breakers IN PANELS only being loaded to 80%.
So what happens if you load a panelboard mounted breaker to 85%? Nothing, if it is not a continuous load, defined as 3 hours or more. If it is longer than 3 hours at 85% of the breaker rating (you have ALSO exceeded the conductor rating by the way), the breaker can possibly over heat and if you are LUCKY, that might cause a "nuisance" trip (nuisance being defined as tripping at a value less than its rating). But I wouldn't count on that because it's more likely you will cause damage to the breaker by heating, melting, or warping the case, which may interfere with it operating correctly.
100% rated breakers are typically only going to be allowed in SEPARATE enclosures or cubicles where there is full air flow around them. There are a few exceptions, such as Sq. D I-Line panels (because the breakers have built-in bus connections that help them dissipate heat, so they were able to list them at 100% in a panelboard). But USING a breaker at 100% also has implications on the conductors that you can use as well. If all elements of the circuit are properly designed however, yes, it will be fin at 100% of its rating indefinitely.
