One consideration not mentioned yet, which is less to do with operating at valid frequencies in invalid voltage ranges (16MHz at 3.3V) but more to do with running at invalid frequencies at valid voltage ranges (24MHz at 5V) is that of heat dissipation.
Every time a gate in the chip switches on or off it dissipates heat. The gate, being made up of MOSFETs, acts like a variable resistor in the period between being ON and OFF, or OFF and ON. That resistor of course dissipates heat. The more frequently it switches the less time there is between switchings for that heat to dissipate out of the chip, and you risk heat buildup.
Ergo, the faster you run, the more heat can build up. That is why PC CPUs have big fans on them - they switch so fast they can't get the heat out of the chip fast enough, so they need help.
The top rated speed of the chip is selected to allow the chip to dissipate its heat buildup reliably under the valid operating conditions (i.e., the ambient temperature, typically max 85°C or 105°C for example). Exceeding that frequency can cause the chip to overheat.
Yes, it can be possible to run the chip faster than intended if you provide some assistance - i.e., a heat sink and maybe a fan, and ensure there is good airflow around it. But of course, on a warm day in summer you may find what was a perfectly working device all winter suddenly starts doing strange things.
Another thing to consider is that of slew rates. Clock signals (and other signals too) take time to rise or fall to their desired level. If the internals of the chip mean the clock signal takes say 15ns to rise from a LOW to a HIGH, and you try and clock it at a frequency where a HIGH period is, say 42ns (24MHz), that leaves only 27ns of valid clock period left. That's just 64% of the clock actually being a clock signal - the rest is rubbish. The same for IO pins. Things like SPI clock outputs will be limited by the slew rate of the IO pin, so if you overclock your chip to get faster SPI you will find things don't always go as planned, as the nice square wave you expect from the clock output isn't square any more.