I have the STMicro USB library configured as a full speed CDC device. This is running on a STM32F405OG chip. Everything works as expected but (very) occasionally it gets into a state where the device NAKs every OUT packet sent by the host, and this continues forever (see attached screenshot of the USB trace). I haven't been able to figure out exactly what triggers this but it seems to happen when there is a lot of USB traffic going through the device. Has anyone seen this before or have any ideas what is happening?