Added in 1994, went unnoticed for about seven years but was discovered only a few months after Borland open sourced the database engine in July 2000. https://www.schneier.com/essays/archives/2001/03/back_door_security_t.html
Back to hardware, more recently some Smart TVs manufacturers were caught with their pants off while spying users watch lists and behaviour using their smart TVs. http://www.wcpo.com/money/consumer/dont-waste-your-money/some-samsung-lg-and-vizio-tvs-now-spy-on-you
And in some cases is even worse than that as devices don't just analyze viewing habits... http://www.geek.com/apps/samsung-reminds-smart-tv-owners-their-personal-conversations-may-be-recorded-1647353/
The risk of getting into an era where every device powerful enough to contain a backdoor will have one is very real; to me the only solution is to push very strongly for Open Hardware while educating users about its importance.