She is allowed to change her mind. its in the rules apparently. This is bad news for anyone who wants to buy her a gift.
If you are a man stressed about getting the right present, here is some great news. The specifics of the gift are less important to her than the meaning she attaches to the item.
The finest gold encrusted powertool, dripping with diamonds would still make her think that you want her to slave for you and would thus be an unsuitable gift.
I found an easy list of great gift ideas for women which gives clues to the underlying meanings that most women attach to presents and so I rarely make mistakes anymore.
If you really want to play safe, no women has ever thrown a diamond or emerald back in a man's face if he has given it in an honest spirit of wanting to please her.