I was testing software on a pixracer using a modified version of the v1.6.5 firmware, and have recently had repeatable hard faults in the system. The fault logs show a full user stack, and an error on the following line.
Type:Hard Fault in file:semaphore/sem_post.c at line: 118 running task: mc_att_control
So far it seems to originate when the ll40ls lidar is plugged in over i2c. Timing is a bit unpredictable but the fault is repeatable.
I have not tried to reproduce this problem on unmodified 1.6.5 software, but the modifications are minimal and i find them unlikely to be the cause.