Cambodian Visas

I feel like this information is hard to find, or just plain unknown.

Foreigners must have visas to enter Cambodia.  You may purchase an e-visa while you are outside of Cambodia.  You can purchase a visa for 1 month, 3 months, or 1 year.  However, we've been told at Cambodian Passport Agencies that tourist visas are only valid for 1 month, regardless of what your e-visa says.  There are other visa options available.
While you are in Cambodia, you can only purchase a visa for a period of 1 month.
While you are in Cambodia, you may apply for a visa renewal only once.
ONE TIME.
There is a $5 fee for every day that you have stayed in Cambodia past the date of your visa expiration. This fee must be paid at your port of exit, be it a border or the airport. 
If you need to extend your stay in Cambodia, you must exit the country.  You may do this by traveling to Poi Pet, which is on the border of Cambodia and Thailand.  There you will exit Cambodia, enter Thailand, and re-enter Cambodia at which time you will be able to purchase a valid visa.
You can also leave the country through the Vietnam/Cambodia border.  It really is up to you. I don't know about Vietnam, but for individuals with US Passports, there is no entrance fee for Thailand.