People can access hacked clients too easily,thus making SMP pvp
hardly legit.
X-ray:
Hard to spot someone with it unless it's obvious tunneling,it makes mining pointless when you/others have x-ray to get straight to diamonds.
Radars:
Impossible to spot someone with them,making an unbush or hiding impossible.
Force Fields/Click aimbots:
Can be hard to spot,someone can just destroy you and you can't know whether or not they did it legit.

What I expected: People playing withouth hacks
What happened: Some people used hacks

How to reproduce:
1)Create a server
2)Look at the players for some days

Possible fix: Do not allow modded clients to join SMP servers

>inb4 invalid