Por si las moscas, las flais

Using this expression will make you sound on top of your Chilango slang. Por si las moscas (purr-see-lass-MAWS-kass) is an expression that everybody uses. Though informal, this expression is not insulting in any way. It literally means "in case the flies ..." and it is used as "just in case."

Some people substitute moscas for flais, the Spanish pronunciation of "flies." Some others use las de hule (lit. the ones made of rubber) in lieu of moscas, an allusion to fake, rubber flies.

Example:

Rubén: ¿Quién trajo el Turista a mi fiesta?
Joselo: Yo, por si las moscas. No sabía si iba a estar tan pinche como la del año pasado.

Rubén: Who brought Monopoly to my party?
Joselo: Me, just in case. I didn't know if it was gonna be as lame as last year's.

Turista Internacional (international tourist) is the Mexican equivalent of Monopoly