Con, contra and en: meeting

1 Con may express meeting of two or more persons in a place or in an opinion.

Me crucé con él en la calle y no me saludó.

Coincido contigo: esto es una vergüenza.

2 Con also indicates stumble with an obstacle that stops the walk.

Tropecé con el bordillo. (= The stumbling forces me to stop.)

With the same meaning contra may be used.

Tropecé contra un poste de la luz. (= And the stumbling forces me to stop.)

If the obstacle does not stop the walk the preposition en is used.

Tropecé en el bordillo. (= And I went on my way.)