Q:

Is it a known scam or honest oversight at Cambodia border crossings to not stamp your entry/exit form?

A month ago I entered Cambodia from Thailand at Koh Kong and was charged an extra USD $7 which is commonly considered to be a scam on the Internet. Koh Kong is considered to be less scammy that the big crossing at Poipet but still fairly scammy.
Yesterday I tried to exit back to Thailand at one of the small border crossings near Battambang that has a good reputation for not having scams. It was the second last day of my visa so I was sure everything was fine. But the officers told me I was missing a stamp on my entry/exit form. I knew I got my entry stamp in may passport because I checked that immediately. But I didn't know I should also check the form.
I tried to get back to Koh Kong before my visa ran out but the distance was too great. I should make it by lunch time tomorrow, one day over.
Could this be another scam that people know of? That they leave off the stamp purposefully knowing the the visitor will get sent back to their crossing and likely now be overstaying? Are they likely to try to charge me more than the official 1-day overstay fee? Are they likely to waive the overstay fee due to the missing stamp being their own mistake? How much should the fee be in 2019?

A:

I'm going to go ahead and guess that the missing stamp was not on purpose in an effort for further extortion, but I can't be 100% sure.
I could not find mention of the stamp being missing for other travellers online like I could find for the extra $7. Worth noting, I also could not find any warnings or advice to check and make sure you got the stamp before continuing on your way into the country. I will edit WikiVoyage and WikiTravel to add this tip.
I had one spare day on my visa when I was told to go back to Koh Kong. That 800km trip took me two whole days and I arrived in Koh Kong fifteen minutes after the border closed at 10pm, which meant that the next day when I arrived at the checkpoint I had now overstayed by two days.
I explained to the officials in military uniform at the departing window that my stamp was missing and I had to travel from Phsar Prum to Koh Kong. A senior officer seemed to understand, nodded, gave a thumbs up and made a phone call. Shortly I was summoned inside where I was given a minor interrogation about why I came to his country and didn't get a stamp, as though it's up to each tourist to reach across and stamp his own paperwork!
But they did not try to get to pay any other mystery fees besides the USD $10 per day overstay fine, for which I was given a receipt. The senior officer who interrogated me also asked how much I had paid when I crossed the border to which I replied that I paid $37 even though I thought it should be $30.
So I do not know if they thought I'd already been scammed sufficiently at $7, whether the officer was concerned that some of his staff was scamming tourists (I doubt it), or what else. His English was more than good enough.
So:

Always check both your visa and your entry form to make sure they were both stamped!
The fee for overstaying remains USD $10 as of 2019.

