I dove into this topic a bit while searching for information about what had gone wrong with my Logitech G700. I traced the problem to the middle mouse button after observing that my keyboard’s caps lock indicator light would come on upon switching the mouse profile over to the one that I use for gaming.
An excellent blog post by hybridsix over at familab (familab.org) first introduced me to the culprit, and one possible remedy. The middle mouse button tactile switch on hybridsix’s G700 experienced a mechanical failure as shown in the photos on that site.
hybridsix used an ALPS SKQGAAE010 from Mouser.com: mouser.com/ProductDetail/ALPS/SKQGAAE010/
Here is a thread from the Logitech forums with other users commenting about failed switches: http://forums.logitech.com/t5/Logitech-G-Mice/Logitech-G700-Middle-button-broken/td-p/502174
Who knows what the rated cycle count is for these switches. Some fail within warranty period, others fail outside of the warranty. There are individuals using mice that evolved out of the MX500 design for CAD work who’ve had to RMA multiple mice because of middle mouse button failures.
A TechReport.com post by Cyril Kowaliski entitled, “A eulogy for my last good Logitech mouse” http://techreport.com/blog/20907/a-eulogy-for-my-last-good-logitech-mouse
Repair guide written for switch replacement on the Logitech G500 (should be applicable to other similar mice): http://pirikukka.blogspot.fi/2015/05/logitech-g500-mouse-repair-guide-for.html
Apparently, Logitech used Omron micro-switches in the past for its middle mouse buttons. They must have made the transition to tactile switches alongside the mouse wheel assembly, which I am certain has gained significantly in complexity. Whether tactile switches are necessary due to the design of Logitech’s modern mouse wheel assemblies, or are being used in lieu of micro-switches for cost-cutting reasons is anyone’s guess.
Google search for logitech g middle button site:forums.logitech.com or logitech middle button site:forums.logitech.com to turn up thread after thread from the Logitech forums.