Ping masking and one of the easiest ways to spot it.

[StarsMine]

For some reason, people dont know this about CSS(or any source game that I know of for that matter), but when you hit tab and look at latency, if someone is <5 they are ping masking. It does not matter where you are, even if you are playing ON the server, your latency can NOT be below 5.

So next time you see someone do such a thing, Ask them to fix there rates, because it does effect game play and it is against the rules. If they dont listen kick them, if they come back without fixing there rates ban.

Proper rates
Rate 20000 or 30000 (20000 is for people running really slow internet)
cl_cmdrate 67
cl_updaterate 67

There is also another way to spot ping masking. When you type ping in console and what comes up is significantly greater then 40 between what is shown in tab and what is shown in console, something is up. This is harder to tell since ping can bounce around rather quickly.
If the tab latency shows 5, ping will show 45 or less. So my assumption is that "latency" it ping -40.

Edit:
Valve be fucking with net codes, Use this as a way to spot potential ping masking, and not an end all be all tool. However dont give me shit in the server when I bring it up either, <5 still should not happen save for a glitch.