A Grave Mistake

Clint and Tyna are having a meal in La Tipica when a woman at a near table makes a number of loud complaints about the people at a close table and about the Indios and about Panamanians in general.
Considering who the people were at the close table, Tyna says she's making a grave mistake to be denigrating those people.
She didn't mean "grave" the way it turned out.