Landman comes back and says," oops I made a mistake 20 months ago so send me the money back! its spent and gone! now what? Thanks for answers

I have imagined this same scenario in some of my past lease negotiations. I have never added any type of disclaimer in my paperwork that would prevent this type of situation. Luckily, this has never happened to me in the many leases I have negotiated. Also, who would be responsible for recovering damages caused by this type of mistake such as the lessor who may have paid taxes on this money from a bonus?