Using a mod called [Inventory Essentials|https://modrinth.com/mod/inventory-essentials] (in my test cases fabric version 21.4.3 [at the time of writing the latest]), which provides "a few inventory control enhancements that have initially been introduced in mods like Inventory Tweaks and Mouse Tweaks" can halt the server when a PlayerDropItemEvent is cancelled when using the Control+Shift+Drop enhancement.

According to this issue] all the mod does is "emulate Vanilla slot clicks".

The server appears to get caught in an infinite loop which I cannot replicate on other server software, only on Spigot.

Reproduction steps:

1. Install Fabric or (Neo)Forge and install the Inventory Essentials Mod.

2. Create a (or use the provided) plugin which simply denies the ability to drop items without the player sneaking.

public class TestPlugin extends JavaPlugin implements Listener {

    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void onItemDrop(final PlayerDropItemEvent event) {
        if (!event.getPlayer().isSneaking()) {
            event.setCancelled(true);
        }
    }

} 

3. Launch a server with the plugin installed.

4. Obtain any item and attempt to drop it using Control + Shift + <Your Drop Key>

5. The server should hang before eventually exiting for running too far behind.