SR flip-flop race condition
On startup, there is a race condition between between Q and Q' settling.  Also, if both S and R are simultaneously toggled active, there is a race condition and invalid state.  However, in normal operation, a race condition is pretty rare.

The case in which there could be a race condition during normal operation (only S or R is active at a time) is when the S or R active edge is not held long enough (minimum pulse width is violated) for the outputs Q and Q' to properly settle.  In this case, there will be a race between the Q/Q' which is propagating through the feedback loop to stabilize the system, and the inactive edge of S or R (whichever caused the toggle).