This can be happened to both sides, to buyers or sellers. As an example of today tradings:
share SEMB.X
pre-open , the first bulk distributed the all the top askers as below.
0.70 200 0.00 09:30:01 N
0.70 600 0.00 09:30:01 N
0.70 300 0.00 09:30:01 N
0.70 300 0.00 09:30:01 N
0.70 300 0.00 09:30:01 N
0.70 700 0.00 09:30:01 N
0.70 500 0.00 09:30:01 N
0.70 600 0.00 09:30:01 N
0.70 700 0.00 09:30:01 N
0.70 2,700 0.00 09:30:01 N
0.70 1,400 0.00 09:30:01 N
0.70 5,600 0.00 09:30:01 N
0.70 1,100 0.00 09:30:01 N
0.70 400 0.00 09:30:01 N
0.70 100 0.00 09:30:01 N
You can see the time @ 9.30.01 - the share is traded at 0.7 . At this time multiple sellers orders are distributed to the number of top askers at 0.70. I know this happened because today also one of my order book, 500 shares been sold at that time. after that there were number of transaction for same amount, but my order not get cleared. it get cleared later on.
To explain this, i am shearing my executed orders list
SELL SEMB.X0000 500 0.70 0.00 346.08 09:30:01
SELL SEMB.X0000 16,800 0.70 0.00 11,628.29 09:48:06
you can compare SEMB executed order book against my records if need.

to make things simple, don't panic if you get sold / buy little amount at the early in the morning and seen the share is trading ta the same value and you didn't get order filled.
