It’s not advisable, but you can overstay your Cambodian visa at a cost of $5 a day for the first 30 days and $6 a day after that, payable at Immigration on your way out of the country.
If you plan to only stay a few days more than 30, this is often cheaper than getting a visa extension. However, be warned that you’ll need exact change in US dollars. If you present them with a $20 bill for a $15 overstay, it’s very likely that you won’t be given change.
The Cambodia government immigration site says that overstayers are also liable to pay the cost of an extended visa, but in practice overstayers are only charged the $5- or $6-per-day fee.
Similar information can be found here
DISCLAIMER - Obviously, there can be further troubles if this gets noted in your passport, so the advice is for you to contact the nearest concerning embassy, consulate or immigration office for the latest up-to-date information. An overstay can harm your chances of getting a future visa for this country, and may do so also for other countries.