Cagar & cagarla

Cagar (lit. to defecate) is used in two forms, cagar and cagarla, both equally rich in substance. Cagar (also cagotear) means to reprehend someone, usually in a nasty way, for some alleged wrongdoing. Cagarla (also zurrarla, defecarla) means to make a big mistake. Do not mix up these two terms—the consequences can be as embarrassing for yourself as humorous for your audience. Use your common sense and let the context guide you.

In the shoe-shine stand:

Shoe-shiner—En la URSS los maestros se la pasan cagando a los niños desde que van al kinder.
Patron—¡No la cagues! La Unión Sovietica ya no existe.

Obviously, the shoe-shiner hasn't being paying attention to the news since the late eighties.

In the operating room:

Dr. Granados—La cagué muy feo.
Dr. Fonseca—Hasta los médicos más reata la cagan en cirugías tan complicadas.
Dr. Granados—No. Cagué muy feo a la enfermera. Le dije que era una pinche ignorante.

Needless to say, Dr. Granados doesn't think very highly of the nurse, though he would prefer to keep his opinion to himself.