Bring in a piece of equipment that worked when it was put in the loft and does not now, and the odds strongly favour an electrolytic capacitor. They are the one component in most designs with a genuinely finite life: the electrolyte dries out, equivalent series resistance climbs, capacitance falls, and the circuit around them starts misbehaving in ways that look like something else entirely.
The symptoms are familiar once you have seen them. Hum or ripple on audio that was clean when new. A power supply that will not start reliably, or starts and then shuts down. Digital sections that fail intermittently when warm. Bulging or vented cans, or a board with the faint smell and residue of leaked electrolyte around the base of a component.
What makes this worth stating is how often it gets diagnosed as something more dramatic. A drifting rail causes strange behaviour three stages downstream, and time gets spent chasing a logic fault that was never there. Measuring the supply rails properly, under load, before touching anything else, resolves a large fraction of these cases in minutes.
The caveat: replacing every capacitor on sight is not a repair strategy either. It is invasive, it risks damaging fragile old boards, and on some equipment original parts are still within tolerance and worth keeping. Measure first, replace what has actually failed or is out of specification, and note what was changed so the next person does not have to guess.