When you are pressing the shortcut to drop items with the mouse pointer, it will also drop your active Hotbar item even when your mouse pointer isn't pointing near the Hotbar.
Videos showcasing the bug:
Minecraft launched without Forge: https://www.youtube.com/watch?v=CMbuc7iAQUA
Minecraft launched with Forge: https://www.youtube.com/watch?v=DMXhskxRKVU
Because of this bug, I dropped my speced out diamond sword into the lava pit when I was throwing stuff out. May it rest in peace.