Two things can happen:
One is stability issues and data corruption. Every chip got some critical paths where the signals take a certain time to pass. If the clock is too fast, the signal may not make it in time before the next clock tick, which may result in data corruption. Lower voltage may further increase instability.

The good news is that most chips have a wide margin of error, especially for low power chips such as ARM Cortex-A8. This means that overclocking the chip by 50% is usually safe. However, this is individual, since some chips have tiny "defects" that do not affect the chip at normal speed but can cause data corruption when overclocked.

The other thing that can happen is that the heat produced by the chip is increased, and as a result, it would die sooner than it would at normal speed. In practice, if you overclock it by around 50% and lower the voltage, physically damaging the chip is much less likely than causing stability issues.