I did network engineering for a gaming company that was having random UDP problems between their game servers and clients. In my first 10 minute conversation with them about the problem I told them it was probably due the client's router most likely Belkin or NetGear. They did't believe me or didn't want to tell customers it was their problem. Two years worth of investigation, troubleshooting, countless man hours, over $100,000 spent in new datacenter hardware and I found out months after I left they identified the problem was one popular model of NetGear router.
A extremely trivial command injection that existed in Netgear routers up until last December. Shown to be WAN-side exploitable with a attacker-controlled website.