Travellers coming overland by bus from Bangkok need to be warned about the stop-over in the border town of Poipet: a place which gets bad write-ups in tour guides such as Lonely Planet. It isn't a town with any inherent attraction, but rather, seems to serve as "hustle-central" for scam artists who see easy prey in backpackers.
The usual crossing routine is as follows. You are first dropped off at an expensive restaurant on the Thai side of the border. Here some of your fellow passengers who have already paid twice the price for a tourist visa which is only $30 have their passports collected from them. You are then told that for $35 your visa can be secured and you are provided with the relevant forms to fill. Here you should politely but firmly refuse and be clear that you want to get the visa at the border yourself. All the passengers have to wait for you on the other side so don't worry about getting left behind.
Some have loudly refused to pay the extra $5 and have been deliberately left behind and had had to pay for a taxi. No one likes being accused of theft. If you are not going to pay the small extra charge (which is a lot of money to these people), do it politely.
Once you go through the Thai and Cambodian visa controls you are then made to sit for perhaps a further hour while various passengers are completing their own visas or while the bus company waits for new passenger to join the onward bus to Siem Reap. If you find yourself waiting for more than an hour you should try and call the Thai agent that sold you the original ticket.
These scams also occur when you book a bus from a "good" Thai hotel or from a Thai agent with a TAT label. Paying above the usual rate of 300 Baht is also no guarantee that you wont be scammed. Should you get scammed report the agent to TAT by email. Another useful idea is to take the contact details of the individual that booked your bus ticket. You may be able to get help at the border if you contact the original seller of the ticket via phone. A warning from you that you will report them to TAT (Thailand Toursit Authority) may get them to help you by speaking with your bus driver/guide.
Also be aware that shared taxis can not take you to your hotel inside Siem Reap and will usually drop you at a Siem Reap bus station or on the Airport Road where you will be forced to take a Tuk Tuk to a guest house that is not of your choice/and or charge you an extra fare.
On the onwards trip in Cambodia you may have to suffer a further stopover at an overpriced restaurant. Count on arriving in Siem Reap after sunset. If you dont know the town well it is a good idea to have to hand the contact number of your hotel in Siem Reap who (if they are good) will send a complimentary Tuk Tuk to pick you up at your bus stop or where the shared taxi has dropped you. You can pick up a Cambodian SIM card for $1-$2 at Poipet (Cellcard/Metfone) and buy a $2 top up card to call your hotel should you need to.
If you are travelling via Poipet, which many thousand do each year (air travel is relatively expensive) then good advice is to do the following:
Poipet is in the view of many people who travel through there a disgrace to Cambodia's young tourism infrastructure. Once you get to Siem Reap you'll realise that Cambodia and Khmer are wonderful.
By and large Cambodia is a safe country to travel in, but in a land rated globally as one of the most corrupt on the planet, little national effort is made to clean up this border town. So long as the scam artists run free (and so long as tourists remain gullible) Cambodia is going to risk hurting one of its main revenue earners in tourism.