Linear-phase EQ
An equaliser that delays every frequency by exactly the same amount, so filtering changes the tone but never the phase relationship between components.
An ordinary EQ, analogue or built from IIR filters, shifts phase around every frequency it touches. Usually that's inaudible, but when the same signal exists in several channels — parallel processing, multiband splits, multiple mics on a drum kit — the small phase differences can produce comb filtering. A linear-phase EQ uses symmetric FIR filters and therefore has constant group delay.
The bill comes in two currencies. Latency can run to thousands of samples, which rules it out while tracking but is manageable in a mix thanks to delay compensation. And the symmetry means the filter responds before the transient arrives: pre-ringing, a faint swoosh ahead of every drum hit that the ear is unusually good at catching. Best on the master bus, worst on drums.