Collisions of prefixes of MD5 hashes in some fixed interval
Just to complete Yuval's answer, there are plenty of collisions if you pick the first 9 digits of the MD5.

For example: 

md5('40236')  = f46657d67 3d95ccf8d12b1075ab7c653
md5('400704') = f46657d67 e7a3de02f64013bfcbd932c


Let $m = 2^{36}$ (nine hex digits), $n = 2^{24}$ (three bytes for an UNSIGNED MEDIUMINT); 
assuming that MD5 is a good hash :), the probability $P(n)$ of a collision is:

$1 - m! / (( m - n )! * m^n) \approx 1 - e^{-n^2/2m} = 1 - \frac{1}{e^{2048}}$

And you'll find a collision on the first 9 digits with high probability ($P(n) \approx 0.99$) every $n = 2^{20}$ strings picked (whatever they are). See Birthday problem on Wikipedia.

So even if you use the binary encoding (three bytes) you obviously get plenty collisions:

Starting from 000000, the first  is:

md5( 0x013701 ) = 8015b8be cf7b0a7e744cd54ac76a302f
md5( 0x0725c6 ) = 8015b8be cf8140319e919766901824bf


Note that 0x0725c6 is a 19 bit number.