  In the last couple of years, Model Driven Engineering (MDE) gained a
prominent role in the context of software engineering. In the MDE paradigm,
models are considered first level artifacts which are iteratively developed by
teams of programmers over a period of time. Because of this, dedicated tools
for versioning and management of models are needed. A central functionality
within this group of tools is model comparison and differencing. In two
disjunct research projects, we identified a group of general matching problems
where state-of-the-art comparison algorithms delivered low quality results. In
this article, we will present five edit operations which are the cause for
these low quality results. The reasons why the algorithms fail, as well as
possible solutions, are also discussed. These examples can be used as
benchmarks by model developers to assess the quality and applicability of a
model comparison tool for a given model type.
