I got this error when I was running:
arcpy.FeatureClassToFeatureClass_conversion(path, gdbpath\featureDatasetA, name)
Where name matches an existing feature Class from a different dataset (i.e. gdbpath\faetureDatasetB). The really weird thing is if you were to run the tool on the correct dataset:
arcpy.FeatureClassToFeatureClass_conversion(path, gdbpath\featureDatasetB, name)
... then no errors occur and the feature class gets overwritten.