I know its not something you manufacture, but the team at Pi Wars asked me to post this which I first posted on the RasperryPi.org forum, concerning the compatibility of a RedBear IOT pHAT (which gives the original Pi Zero a wireless capability without using a USB wireless dongle) with code such as Servoblaster. Unfortunately, the Servoblaster code by default drives some of the pins which the pHAT needs for talking to the wireless chip. These advanced compatibility problems are quite concerning, because the Pi Zero becomes almost unusable until the servoblaster service can be stopped :
"My RedBear IOT pHAT works OK, but of course has now been superseded by the PiZeroW. I am trying to use servoblaster on my PiZero, and the default setup causes constant interrupts that use over 95% of cpu time on irq/86-mmc1. Managed to stop the service (not quite sure how), and then a reboot got everything working. Need to set the used GPIO in servoblaster so it doesn't upset the PHAT when it starts up from initial boot."