[Mp4-tech] [H.264 Video] DPB and DPB bumping

[Tomo]

Hi
I have questions regarding DPB and its bumping.
Standard specifies max DPB per level.
HRD assumes buffer management (CPB, DPB) is done instantly (meaning
zero delay), but actually it is not possible (for physical
implementation).
Now, in case DBP, DBP needs to be managed as described in the
standard. 
They say for actual decoder design, decoder needs to have 2xDPB
to handle worst case DPB bumping. I guess it could happen at the
boundary of IDR, perhaps because IDR makes DBP reset, but
all the frames in DPB have not yet been displayed and those
not-yet-displayed frames need to be saved outside of DPB.
Is my understanding correct?
If yes, then can this extream case actually happen?
I mean within standard  and actual (well, some pathological
encoder might make such case...).
Any comment?
Thanks
Tomo