5 sourced reports, with what to check before you buy. Each one links to the page it was reported on.
Costly or dangerous
Earcup breaks off the headband yoke on the Momentum 3
Found via 'Sennheiser Momentum 3 earcup hinge broke headband crack'. A Russian headphone forum user researching the Momentum 3 reports that on this exact model the earcups break off the headband (чашки отламываются), and that the size-adjustment mechanism loosens over time.
What to check. Inspect the earcup yokes and sliders before buying; a broken yoke is a structural, likely non-repairable failure that ends the headphone's life.
Fold-to-power hinge microswitch fails, draining the battery overnight
Found via 'Sennheiser Momentum 3 common problems'. A repair/used-buyer guide for the Momentum 3 says the automatic fold-to-power hinge switch is prone to mechanical wear; if the internal microswitch wears out or gets debris, the headphones stay powered on in their case and drain the battery overnight.
What to check. Test that folding the headphones shut actually powers them off; a worn hinge switch silently kills the battery when stored.
Analog cable port switch sticks, leaving the headphones stuck in airplane mode
Found via 'Sennheiser Momentum 3 Bluetooth connection drops firmware issue'. An Ars Technica forum owner describes that inserting the analog cable actuates a switch inside the 2.5mm port that turns Bluetooth off; after common use the switch stuck, leaving the headphones in 'airplane' mode with Bluetooth disabled and no pairing.
What to check. Use the analog cable sparingly and follow the twist-to-lock/unlock instruction; a stuck port switch disables Bluetooth entirely.
Headband slider loosens and slips in length over time
Found via 'Sennheiser Momentum 3 headband hinge broke forum'. A Galaxus owner review (in German) advises testing right after purchase whether one headband side is slightly loose, because it does not bother at first but over time the slider keeps slipping in length.
What to check. Check both headband sliders hold their position; a loose slider makes the fit unstable and worsens with use.
Earcups come loose; community prints a slider/spacer fix
Found via 'Sennheiser Momentum 3 earcup hinge broke headband crack'. A Printables 3D-print model is explicitly made as a fix for the Momentum 3 earcups coming loose, corroborating the earcup attachment looseness reported elsewhere.
What to check. If earcups feel wobbly, a printed slider/spacer is a cheap workaround, but it signals the yoke fit degrades with use.
These reports come from the pages linked under each item: owner forums, repair guides and workshops. They are not a diagnosis of any one unit, and a careful check of the one you are buying still applies.
Text from the Kriko Over-ear noise-cancelling headphones catalog, version 0.1.1, under the CC-BY-SA-4.0 license.
