The most common mistake I have seen at IKEA is that people underestimate the size of their car.
This means that when you get flatpack furniture it may not fit inside your car - at least not with you and a passenger too ...
No seriously - people do forget that they have to be able to get into the car and drive it

(Thats why you often see Police cars near IKEA (you thought they were smartening up the police cells ? No they are waiting for an easy booking of a car being unsafely
laden.)
The trouble with IKEA is that it is SO tempting you think thats accidental ?- no - just a great bit of marketing !)

The other mistake many people make is to park too far from the loading area .
Then you have to carry big stuff to your car - not a smart move.
Take a friend with you and get them to wait in the loading area while you get the car or van ( really clever people get a friend with a van to go shopping with them .. I must try that sometime !)