Bukkit

When buying a crackshot weapon from a display shop, if it happens to go in the currently-selected empty slot, it will fire the weapon as well.

Actually this should already happen. I optimized some things in the method where I am handling the PlayerInteractEvent, but I don't think that is changed the cancel behaviour. If it does, good for you, mysterious to me.
Whats the name of the plugin that provides the special item?

Crackshot, which is not open source unfortunately but idk if it's obfuscated. http://dev.bukkit.org/bukkit-plugins/crackshot/

Hmm, still happening with the latest build. Probably is listening to arm swing event?

:/
The only two places where an PlayerInteractEvent is handled:

• https://git.tallbyte.com/blob/ShowCaseStandalone.git/master/src!com!kellerkindt!scs!listeners!PlayerListener.java
• https://git.tallbyte.com/blob/ShowCaseStandalone.git/master/src!com!kellerkindt!scs!listeners!SignListener.java
  
  Both cancel the event appropriate (just tested with a debug message)

This is actually reproducible with just vanilla items as well - e.g. right click to buy a minecart with empty hand = buying the minecart, and then immediately placing it down.

This is actually reproducible with just vanilla items as well - e.g. right click to buy a minecart with empty hand = buying the minecart, and then immediately placing it down.

This is actually reproducible with just vanilla items as well - e.g. right click to buy a minecart with empty hand = buying the minecart, and then immediately placing it down.

This is actually reproducible with just vanilla items as well - e.g. right click to buy a minecart with empty hand = buying the minecart, and then immediately placing it down.