I was recently re-evaluating our back-up procedures and discovered and found a nasty bug with the arcpy’s ListFeatureClasses request. If you have a feature class in a feature dataset with the same name, ListFeatureClasses may not find it or anything else in that feature dataset. Unfortunately, we recently made our daily backup a python-based system that uses ListFeatureClasses and got bit by this bug. After discovering missing data in our backups, I reconstructed what happened and found this bug.