Your discarded smart lightbulbs reveal your wifi passwords, because they are stored in the clear.
Not to mention, someone replacing your bulb and getting the password that way.
Yes, I know, the spy might not program the replacement bulb to operate like the old one. Dead give-a-way, right? My bet is that you'll think the bulb just forgot it's settings, or not notice at all.
This hack was publicized here, and originally researched here, if you want to know more.
Thanks to our Blue Blaze Irregular from the Jersey shore for this one.