I'll give you one example (but not surety):
I went to book for 12649 (BPL- NZM) on the day 1 of ARP. In the morning I saw it 52 available in SL class. I went to book at a PRS at the evening (as I took lesson from my previous experiences from IRCTC). When I reached the counter, it was down to available 42. I asked for SLB and got it.