Is this common?

Hello. I recently decided to try to overclock my memory since I play a lot of DayZ and ARMA2 and there is a bit if a benefit from faster RAM according to benchmarks.

My RAM is G.Skill Ripjaws X 2x4GB DDR3 1600mhz. stock timings are 9-9-9-24, voltage is 1.5v

So I did little baby steps. First 1866mhz at 10-10-10-28 at 1.5v. memtest86+ showed no errors after 5 min. So I decided to, why not, try to go to 2000mhz. I set the timings to 11-11-11-30 and voltage to 1.54v. I go into memtest86+ and it turns out that I accidently set it to a whopping 2133mhz. But i decided not to stop it, and it went on with testing. Its been sitting here for 15min without any warnings or errors... people say that if unstable, it would give errors almost immediately, i have had none so far. i just think it is so weird that it is stable at such a ridiculous clock... so my question is: is this normal? or am i doing this completely wrong?

It's pretty common as many lower clocked kits have the same memory chips as these 2133+. The main difference is that higher clocked kits were tested so manufacturer guarantees higher frequency.
Most new IC are overclocking to at least 2000-2133. Hynix/Samsung go higher while Micron/Nanya usually won't go past 2133.

Depends from memory chips your memory can probably run at 2400 ~1.6-1.65V and timings like 10-12-12 or 11-13-13 or something similar. You can try and maybe it will work.