Yes even 'simple' devices these days will have devices (ADC/SPI etc) running in parallel often using DMA, multiple semi-independent clocks, possibly nested interrupts etc. Oh and the UART for some reason always, always has bugs, so hopefully you're using multiple levels of error checking.
Yeah, it was a "fun" surprise to discover the errata sheet for the microcontroller I was working with after beating my head against the wall trying to figure out why it doesn't do what the reference manual says it should do. It's especially "fun" when the errata is "The hardware flow control doesn't work. Like, at all. Just don't even try."
