Beyond a practical use in code review, source code change detection (SCCD) is
an important component of many mining software repositories (MSR) approaches.
As such, any error or imprecision in the detection may result in a wrong
conclusion while mining repositories. We identified, analyzed, and
characterized impressions in GumTree, which is the most advanced algorithm for
SCCD. After analyzing its detection accuracy over a curated corpus of 107 C#
projects, we diagnosed several imprecisions. Many of our findings confirm that
a more language-aware perspective of GumTree can be helpful in reporting more
precise changes.