Sets that add fatigue without adding adaptation, either because they are too easy to stimulate anything or because they sit past the point of useful returns.
Two kinds. The first is sets stopped far from failure: three sets of 12 at RPE 5 recruit little and cost real time. The second is sets beyond the point where more volume stops helping, which for most trained lifters is somewhere above roughly 20 hard sets per muscle per week.
It is hard to spot from the outside because the session looks busy. The tell is in the log: total sets rising week after week with no matching rise in load or reps.
The fix is to cut, not add. Drop the last two sets of each exercise, hold everything else, and see whether the numbers keep climbing. Most of the time they do, in less time.