Revenue seat blocking. For example, I regularly find 12A/K on regional SIN flights available last minute, when I tend to book, often as the last window seats available and otherwise I'd be sitting in a middle.

Or a recent flight when the F cabin section was blocked off but opened to me solo (J class flight) when a few seats in rows 11-26 were still open, and it was apparently done "automatically by the system" according to the agent. (I had asked for rows 11-12, which I was told were full, but it didn't matter since the system put me in row 1 anyway).