Do Not Alter Default Packages
Do not ever recompile or add resources to packages that come in a base install or a bonus pack of any Unreal engine game such as UT or UT2003.
This means:
- don't add textures to the default texture packages
- don't add sounds to the default sound packages
- don't add static meshes to the default static mesh packages
- don't modify the files included in the UnrealScript sources and recompile the packages.
- ... and similar things
If you've done any of these (or other similar), STOP READING THIS AND REINSTALL THE GAME. Right now. We're serious.
Why???
Each time a package is compiled, a checksum is generated. When playing over a network, the packages on the server are compared against the packages on the client – the idea is to insure that everyone has the same packages and is therefore playing the same game. Recompiling the stock game packages will create packages with different checksums than what most online servers and clients will expect... and while the game may still run fine, the modified packages will prevent the player from connecting to another server, and prevent clients from connecting to the player.
But then how do I write new code?
Make a new package and Create A Subclass.
Related Topics
Wiki writers: link to this page whenever you need to warn people of this!
El Muerte TDS: does it even need text ? it's just a DON'T.
Tarquin: yeah. but everytime it's explained, it takes up a paragraph or so to explain. Easier to link to this page and put the paragraph or so here.
Foxpaw: Yes, plus if you just tell people "DON'T," they'll ask why. Alternatively they may just try it and find out. Eve and Pandora: Don't be like them! 
