An interesting Mae Sai border crossing experience: this border is notorious for easy peasy border bounces but my gf is a Hong Kong passport holder and was grilled heavily doing her first ever border bounce and was Asked to show onward travel. (of course it's not unreasonable to ask but in my years here I've never heard this asked for at this crossing)
2 sets of supervisors called to review. Only when I (USA passport) started talking for her did things move forward. Immigration told her "only this time" even though official documents that circulate state that Hong Kong is allowed 2 per year. From what I could understand, I believe the confusion for them was the difference between a mainland China passport and a Hong Kong passport. Luckily, we were prepared and have legitimate travel booked. Made it through after about 20 minutes at the immigration desk.
So as always, be prepared!
TLDR : Answer Summary
The user shares a detailed experience of crossing the Mae Sai border, particularly for their girlfriend who holds a Hong Kong passport. Contrary to common beliefs about easy border crossings, she faced tough questioning regarding onward travel. The situation escalated to involve supervisors, highlighting confusion over the treatment of Hong Kong passports versus mainland China passports. After presenting legitimate travel plans, they were allowed passage after 20 minutes. The post emphasizes the importance of being prepared for border procedures, especially regarding the specific rules for different passport holders.