E.g., the Macintosh launched with a hard-crash bug in the Clipboard code in ROM [1]. When you're struggling to meet a tough date for a huge project, things fall through the cracks. They fixed it later with on-disk software.
[1] http://www.folklore.org/StoryView.py?project=Macintosh&s...
Look down that forum post for all the people asking for help getting it working. Every one of those would have been a lost sale if this were in the shipping product.
And who knows: maybe this was off for a reason. Maybe they hit some voodoo late in the process which produced a crash bug on one of their 19 test systems that didn't occur with a debug build. So one of the engineers tries an unoptimized build and it works. Slightly slower is better than crashing, so they pulled the trigger and shipped it.
Shipping software only looks easy when you look only at code.