It is not a mistake. "Had" and "have" are equally correct, but the meaning changes slightly. You have the option of using subjunctive mood ("had"), but you do not have to.
Here are some examples:
If I swim tomorrow, I would rather have dinner with Sally.
I Jack comes to the party, I would be surprised.
You have the option to use subjunctive mood if you wish to convey that something is hypothetical or counterfactual, but you are not forced to convey that if you do not wish to convey that:
If I swam tomorrow, I would rather have dinner with Sally.
I Jack came to the party, I would be surprised.
Only the speaker can decide whether or not to use the subjunctive mood. Grammar books can explain what it means to say it one way or another but cannot make the decision. It's not a question of being right or wrong. It is a question of what you want to say.