The reason for asking is that on a recent trip to AMS the B seat in the first row of ET was blocked in the seat map but on boarding the middle seat was occupied. According to BA Source the A320 that operated the flight had space saver seats. Is this the only aircraft where this occurs?

Travelling to JER with my partner on a 737 next week and don't want to reserve the first row of ET to find someone in middle of us on boarding.

Thanks