These days I had to deal with a web service which needs a parameter as MD5 hash. I played around with for some time but authentification always failed, I did everything according to the documentation, however. Then I found out that the other service is checking the hash value case-sensitive, so I had to add a lcase to the string.
Some further research shows that almost every other service I know produces lowercase only hash values, ColdFusion produces uppercase values. Be aware of that! ;-)