John Lim has posted about an interesting bug he's come across when working with the output of two strings that should look the same:
Someone reported a bug in ADOdb, the open source db library i maintain. I went crazy for half an hour until i realised the problem.
According to him, "if you expect the above code to produce the same values, you are sadly mistaken". His example gives an interesting result for the first echo statement - not echoing the 9 in the first character like it seems would make sense. Check out his post for the code and try it out for yourself.