Inside vs outside miters
An outside miter wraps around an external corner of the house — the corner points away from you. An inside miter folds into an internal corner, such as where a wing meets the main structure. Inside miters carry more water, because both runs feed toward the joint rather than away from it, and they leak more often as a result.
Why the corner is the weak point
Everywhere else on a seamless run the gutter is one continuous piece of aluminum with nothing to fail. At a miter, two pieces meet and the only thing keeping water inside is a sealed joint. Add turbulent flow as water changes direction, plus twenty years of Kansas City thermal cycling working the joint open and closed, and the sealant eventually loses.
How to spot a failing miter
Look during rain, not after. A failing miter drips from the underside of the corner, or runs down the outside face rather than staying in the trough. Between rains, the evidence is a dark stain on the fascia below the corner that never quite dries, and often peeling paint or soft wood directly underneath.
Repair, not replacement
A leaking corner does not mean you need new gutters. It means one joint needs to be properly re-made. Running a fresh bead of sealant over the old one is the patch that brings us back the following spring — new sealant will not bond to degraded old sealant. The repair that lasts means opening the joint, cleaning both faces to bare metal, and re-sealing from the inside so water pressure holds the seal shut.