This is the first time I have heard of this.
The Guanajuato mummies were discovered in a cemetery of A Guanajuato Mummya city named Guanajuato northwest of Mexico City (near Léon). They are accidental modern mummies and were literally “dug up” between the years 1896 and 1958 when a local law required relatives to pay a kind of grave tax. You could pay the tax once (170 pesos) and be done with it; this option may have appealed to wealthier individuals. But you were also allowed to pay a yearly fee (20 pesos); this would have appealed to less wealthy families. However, if the relatives could not pay this yearly tax for three years, the body (which had, by the way, become accidentally mummified) was dug up from the cemetery and (if the fee still wasn’t paid) placed on display in El museo de las momias. [Of course, what if the person's family had moved from town--or what if the person was the last person from their family? Well, it didn't matter; the law was the law!]
Some freaky pictures of the mummies are here.