As we have seen in the past there have been a few. World Cups are known for this and this happens in rugby, soccer and cricket.
I'm not sure how one terms "upsets" as they are generally known because there is always is always a possibility even against the biggest odds. Maybe "wild cards" would be better.
I'm not refering in this context to the top winner but who qualifies to go through this first stage
In Pool A, France and NZ are the obvious selection but could Italy spring a surprise, maybe if the other are caught napping.
In Pool B, there is the biggest risk because Scotland can stand up to the present "giants"Ireland and SA. Even Tonga could upset the applecart but this is a real 50-1 "roughy" as they say in horse racing.
In Pool C too, it's quite possible that Georgia (probably the very dark horse as their rugby is growing in leaps and bounds and are brutes in the scrum) could unseat Wales, as they did so quite recently. Australia however won't be beaten by both...in fact I think the Wallabys are well "placed"but even Wales who's struggling a bit at the moment could beat them.
Pool D , anything could happen, it's not simply between England and Argentina...Japan has the capability of upsetting Argentina and even England. The Boks learnt a lesson from this 2 WC' s ago.