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

My question is, how common is this problem in the leasing process? This is first time on this forum that I have read about this particular situation occurring between a lessor and lessee. I would think that the landman on the other end has conducted an extensive research of the records in order to avoid this type of mistake.