I was trying to combine many feature classes from one geodatabase into one feature class. All input feature classes have the same schema and all features are line and have the same attributes. The total number of line features is big (~ half million). I noticed quite a bit of features are missing after merging/appending. I tried both scripting (arcpy merge_management and append_management function) and using merge and append tool in ArcToolBox, I even tried to repair geometry first before merge, define extent before merge..,none of these seem to fix the issue. When I changed the computer, the missing issue still existed, just the number of missing features are different. Any suggestion or did anyone here experience this before? Did I miss anything when using merge/append? Thanks.