As integer overflow is considered in solidity as one of a huge problems, in real life overflow happens rarely, actually because the values causing overflows are pretty big. However, if you use uint variables underflow is a much more critical problem that can happen frequently.
contract testContract
{
uint public integer = 0;
uint public sub;
function minusOne() returns (uint){
return integer -1;
}
function checkSub(uint balance, uint amount2) returns (uint){
sub = balance - amount2;
require (sub > 0);
return sub;
}
}
In the previous example calling minusOne() does not throw an error message, instead it returns with the biggest possible uint value: uint256: 115792089237316195423570985008687907853269984665640564039457584007913129639935
Similarly in the checkSub() function if you the amount2 is bigger than the balance variable, sub variable will underflow resulting a very huge integer number which will not trigger the require condition.
