Some years ago I was working on the replacement version of a big product. We had some of the leading clients using the new version while most were still using the old version. There was a need for a new piece of functionality that would eventually be used by both systems but be introduced into the older system first (as clients get what they want).
To make life easier we asked the team building it to build the new utility as a distinct product that would be integrated via an API. This would have made moving it over simply a case of integrating the API into the new system.
We made the mistake of letting the team define the interface.
A year later when we came to integrate it we found the following contract:
interface ITransfer {
string action(string input)
}
Needless to say it was not a simple replacement job. Every call used the same interface and it was internally mapped to whatever we needed to do. Sometimes it was XML other times list of integers.
Lesson learned: Watch out for overly generic contracts, they are as good as having no contract at all.
Developer Rants 