I have never had a problem obtaining a Residency Certificate, that is until I came to live in Phnom Penh!
Living in Battambang many years ago I needed the residency certificate to open a bank account, at the time I only had to do very little because my landlord sorted it out for me.
When I moved to the north of the country along the banks of the Mekong River, the borders of Cambodia and Loas actually cross if you like, there was more Lothian than Khmer living in areas of the riverbanks. I was informed that the police, to do with immigration authorities were going to pay me a visit, this had come about because the locals had reported me has being in the area. The police had driven 200km from Preah Vihear to my location, they took photos of my passport and visa, explained to me they were worried first off of my safety being so remote, but now they would register me being there, they had some food and refreshment and set off on their journey back. The local police a few days later issued me with the appropriate residency certificate. Not only that it came with benefits, such being I could cross the river by ferry into Loas to the market town (like the Khmer residents in the area) no visa required (but only a day visit, not allowed to stay overnight}. If I wanted to take my own boat for a 7-day trip along the river, I had to get a permit from the local police, they would have it authorised by the Loas police and given to me two days later, cost was 10,000reil, no visa once again needed, but I could not stay on the mainland overnight, islands were permitted.
Then I moved to Koh Kong, I needed a new residency certificate for my work permit, it was easy and straightforward from the Sangkat. I could never obtain a pass like the Khmer have here to enter Thailand and travel the limited distance to Trat, but I know of a person who did, took some work for him with officials Khmer and Thai, and costs.
Now in Phnom Penh, I took the list of documents that Freightdog has listed in his post above, plus my old residency certificates with me to the Sangkat, it was Friday, I was told to return on Monday, due to them having more time. Monday came and they told me "can't do". The landlord sent his sister to the Sangkat on his behalf, she returned explaining to me that all has changed due to they don't have to register you anymore due to the new introduction of the FPCS App. I tried again at the Sangkat, only to be escorted out of the building, after they showed me a document I had to obtain first before they could issue a residency certificate. Has Freightdog rightly posts from the Ministry of Justice, they told me it was $100, and once obtained the Sangkat will want their payment. Now I actually only wanted the residency certificate so I could get a exchange driving licence at the time, and after all this I approached an agent who obtained my Cambodian driving licence on my behalf for the cost of $80.

I've had to supply a residency certificate with my application to extend my annual visa for a handful of years, ever since I started doing the extension myself. But I think that's because I do not use an agent and keep the $120+ extra slush fee myself, rather than give it to the agent as many others prefer to do. If you use an agent, the agent will kind of cover that requirement for you.