I was just thinking of starting a thread here today on this! Here's my "ordeal" (TLDR: I was held up, probably illegally by international standards, at PEK for well over four hours, barely made the flight.)

Traveling HKG-PEK-YYZ on CA + AC with a 5 hour layover in PEK on a Canadian passport (but with the place of birth saying "Hong Kong")

Arrive in PEK and am interrogated (in Mandarin, which I do not understand at all) for 3 hours. After giving them blank stares and honestly saying "I do not understand you" they find someone who speaks a little broken English and get the point across that they simply do not believe I can be born in HK and am therefore not a Chinese citizen. They also believe that I'm lying about not understanding them, because apparently "all HK people understand Mandarin"

I'm then forced to give a detailed history of how my parents and I emigrated when I was two months old, where I went to school, where I grew up, etc. etc. They'd repeat all the questions in Mandarin (I think), which I could only deduce as a ploy to catch me off-guard.

In Round 3, they made me take out my passport and asked me things like why I was traveling in business class if I wasn't on business to HK (as if a high school student has "business" over Christmas vacation...), then questioned why a high school student was in business class in the first place. After a session of half-English, half-gestured communication, I'm made to sign a blank piece of paper numerous times to ensure that the signature on my passport matched the ones I was signing. (I almost forgot to take that blank piece of paper with me... I cannot imagine what they could use with my signature if they wanted to! )

I am finally given clearance with a 72-hour transit visa stamp with slightly over 45 minutes before departure, and am given further grilling by security for looking too sweaty and nervous. I ended up being the last person on the plane and delaying the departure time by about 20 mins, with visible glares from virtually everyone as I walked down the cabin.