This bug was discovered in version 4.0-460. I discovered it while playing the most recent release of the Better Minecraft 4 (v36) modpack.

• Obtain Flamarang/Pickarang/etc
• Fill every inventory slot except for one
  • Empty slot for rang
  • You can put 1 cobblestone in every slot
• Find bedrock (or any other unbreakable block)
• Right-click and throw your rang at it
• Your rang should return to your inventory, but a new rang will also have been tossed onto the ground
• Free up a single item slot in your inventory
• You should pick up the duplicated rang
• You now have two rangs
• This can be repeated infinitely as long as you're throwing it at an unbreakable block while having a full inventory.

To follow-up:

• This ONLY works on unbreakable blocks like bedrock or reinforced deep slate. It doesn't work on blocks that just cannot simply be broken by a rank (i.e.: obsidian, ancient debris, etc.)

When the pickarang hits a block it temporarily puts its item form in your hand, this way you "mine" the block with the pickarang and stuff like the silk touch enchantment works as expected. This is supposed to happen all within a single tick; looks like it's not properly removing itself from your hand after you try to mine the block.