The bug

The string representation of NBT integer arrays always has a trailing comma after the last entry. This is different to NBT lists.

How to reproduce

1. Use the following command

   /give @p stone 1 0 {IntArrayTest:[1,2,3,4]}
   

2. Throw the item on the ground and inspect its NBT data

   /entitydata @e[type=item,c=1] {}
   

   → You will see that the last item in the integer array has a comma behind it

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.nbt.NBTTagIntArray.toString() adds a comma even for the last item.