What if a capacitor exposed to slightly higher voltage than rated?
For critical (product design) applications: check the datasheet and ensure in your design that the voltage across the capacitor doesn't ever exceed the absolute maximum rating.
For non-critical applications (e.g. prototypes, testing): try it and see what happens. A bit overvoltage is usually going to be OK, since there will be a certain safety margin built into the design of any capacitor. When overvolting capacitors from sources that can potentially source a lot of current, be sure protect yourself and your environment against shrapnel from exploding caps.