No, I'm not talking about old age pensioners!

Mehdi, simply put, if you violate the manufacturers specs, then expect inconsistant results. In the case of an input, as Erik says it will register as either a '1' or a '0'. If you're in the 'grey' zone the input may register as a '1' sometimes and a '0' others. I think you understand this. So the question is "how do we cope with this?". Obviously we try to avoid this situation but in the real world this can happen. Things like PLCs have software to 'debounce' the input - the input must be in a particular state for a given period of time to register one way or another. So yes, we would normally use a timer. Do a search on 'debounce' and I think your question is answered.

As a general rule, if you violate input voltage specs or timing specs, the circuit will become erratic and normally temperature sensitive. The manufacturers specs are based on the design, statistical sampling and temperature to guarantee to us, the user/designer a safe area to design within.