snappy-java contains a bug that can lead to situations where separate SnappyOutputStream instances end up sharing the same input and output buffers, which can lead to data corruption issues. See https://github.com/xerial/snappy-java/issues/107 for my upstream bug report and https://github.com/xerial/snappy-java/pull/108 for my patch to fix this issue.
I discovered this issue because the buffer-sharing was leading to a test failure in JavaAPISuite: one of the repartition-and-sort tests was returning the wrong answer because both tasks wrote their output using the same compression buffers and one task won the race, causing its output to be written to both shuffle output files. As a result, the test returned the result of collecting one partition twice (see https://github.com/apache/spark/pull/5868#issuecomment-101954962 for more details).
The buffer-sharing can only occur if close() is called twice on the same SnappyOutputStream and the JVM experiences little GC / memory pressure (for a more precise description of when this issue may occur, see my upstream tickets). I think that this double-close happens somewhere in some test code that was added as part of my Tungsten shuffle patch, exposing this bug (to see this, download a recent build of master and run https://gist.github.com/JoshRosen/eb3257a75c16597d769f locally in order to force the test execution order that triggers the bug).
I think that it's rare that this bug would lead to silent failures like this. In more realistic workloads that aren't writing only a handful of bytes per task, I would expect this issue to lead to stream corruption issues like