This should be sticked.
The crossfire developers keep adding more and more server-side checks. This will cause in more client errors. First of all some trivias:
1.Client errors are unpatchable without hacking the crossfire servers. So please don't ask to fix them!
2.Some hacks will cause instant client errors and can not be used anymore.
3.Other hacks will only cause client errors if you use them too often (abuse).
The following (incomplete) list shows you the error codes and what functions may cause them:
1.
•Caused by: Speed Hack
•Notes: Speed Hack works without any error up to 120% - 125% of the normal speed.
2.
•Caused by: C4 plant & defuse Hack
3.
•Caused by: Knife Range Hack
•Notes: Causes error after 1 kill however you can make it undetected. //Credits - Abel09
4.
•Caused by: Attack Speed Hack, Fast Reload
•Notes: Don't abuse the fast reload hack. Careful usage works without an error.
5.
•Caused by: Shoot through wall Hack
•Notes: Works fine for the NA version, but causes client error in Crossfire EU.
6.
•Caused by: Animation Hack, Master Ghost
7.
•Caused by: Escape Hack
8.
•Caused by: Flash bang Hack
9.
•Caused by: Damage Hack
•Notes: Damage Hack works fine with a value of ~120% for the most weapons.
10.
•Caused by: Player position & movement Hack, Teleport Hack, Super Jump, Fly Hack, Glitching
11.
•Caused by: Grenade Type Hack
12.
•Caused by: Hidden Hack
13.
•Caused by: Sniper Weapon Hack
14.
•Caused by: Rifle Weapon Hack
I will update when I have the time

There are more errors. If you know anymore then just reply back to the thread and I will update it.
Credits:
Saedelaere
Abel09