The Bug Nobody is Allowed to Understand
In the 1980s, proprietary software users discovered the problem of the bug that nobody is allowed to understand. When a problem occurs in the interaction of multiple proprietary software packages with different developers, none of them is allowed to study the source code of all the pertinent programs. As a result, none of them can understand the bad interaction between them, and the bug is never fixed except by accident.
According to this article, a similar problem now occurs between multiple instances of Software as a Service.