Stayed at hotel for 7 nights. Upon check-in, I witnessed house-cleaning showing stained linen to reception but didn't think anything of it. Many times, I noticed many plastic bags at reception, labelled with room numbers and Thai writing on the bags. A few days into my stay, reception staff opened up a plastic bag with my room number on it and pulled out a towel with a small stain.
She said it had been laundered and the stain could not be washed out! Because of this, I needed to pay 350 Thai Baht. I asked her, how did she know this towel was from my room, when all towels from all rooms are laundered together in the same wash cycle. She then replied in broken english that house-keeping had tried to rub the stain on the towel, between their hands and it could not be removed. I told her that I didn't have the money on me and to give me the towel back. I then went back to my room and washed the stain out with hand soap and hair shampoo. No more stain!
I went back to reception and she checked over the towel and I didn't have to pay the fine.
Talking to other Australian families, they had the same problem. One couple, were fined for a stained pillow case. Another couple had a phone call from reception that their room was untidy /dirty while they were still staying at the hotel.
You pay 500 baht for your room key. Upon check-out, they check your room before returning money.