It's probably possible to "unknowingly" book an exit row seat when you don't qualify. My friend and I are paramedics so we would have no problem sitting in an exit row and meeting the qualifications. Honestly, I prefer to sit much closer to the front of the plane because I feel a little too claustrophobic in the middle of the plane, but in an emergency, that all goes away. Anyway...we buy our tickets on release day which is quite a bit in advance of travel. A few years ago, we booked our flight and before our flight came around, my friend ended up having shoulder surgery and she was travelling in a sling. Obviously, she would not have been able to travel in an exit row seat, but she didn't know that when she booked her ticket. If she had booked an exit row seat (not possible with SW at the time), we would have had to switch seats when we travelled.
So, the TLDR version is, yes, I would say it is possible to inadvertantly book exit row seats and not be qualified

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