"Un marrón" = A brown*

Obviously, we are not talking about a colour here, but a problem or difficult situation. The idiomatic expression people use is "comerse un marrón" (To eat a brown*) when they want to say they have to face a problem and more often than not, it is somebody else's. If the problem is definitely a big one, go for "marronazo".  Another slang expression is "patata caliente".

"¡ Madre mía qué marrón! ¿Qué hacemos ahora?"
"Yo paso de comerme sus marrones"
"¡Esto es un marronazo, tío!"