Creative mode severe exploit with packet 107

If you're in creative mode in multiplayer, the server will let you can give yourself items with arbitrary custom NBT data by modifying the itemstack in packet 107. This is a problem because certain items with bad data can crash peoples clients or even the whole server.

Here's two examples of bad itemstacks I found:

1. A firework with an "Explosion" tag compound that's missing a "Colors" integer array will crash every client within render distance.

2. A potion with a "CustomPotionEffects" tag compound will crash the server and corrupt the world (because it expects a tag list not a tag compound). I tested this on a local server. The idea of the wrong tag type probably works on other items too.

Basically if you're in creative you can crash a server. This could potentially be a huge problem if more people find it.