How to debug ATPG fault simulation mismatch?

I'm doing fault simulation using ATPG pattern and netlist, but a lot of mismatch are reported, these mismatch are all about one scan cell, such as:

DPV: Parallel Mode ScanCell dut.bus_intf.apb_sync.r_req_reg_0_ expected to be 0 was 1
Detected at time 1749000
With WaveformTable "_default_WFT_"
STIL index 285 of chain 3,
TetraMAX pattern 7 (detected during load of pattern 8), TetraMAX scancell 432

Below is my waveform which the first mismatch report:


My questions are:
1. in the error time(which is indicated by a white line), why Q is changed before CK? I'm running simulation with no delay (no sdf annotated).
2. why Q changed in the time indicated by a yellow line ? I think it should change with posedge of CK, but it not. It's so curious....
3. How to debug this mismatch? Generally what's the root cause of this kind of error?

Thanks very much!!

Hi Xiaojigao,

If you got the answer for this question, Please let me know the root cause for changing the Q before rising of the clock.

Waiting for your reply.....