stumble across something—find smth. by accident:
- Not too long after seeing the movie, I stumbled across the collection of short stories from which the movie got its start.
stumble over something—
1. hit the foot against smth. and almost fall:
- The pony stumbled over a stone in the path, and unseated his rider.
2. say words awkwardly:
- They seemed to have trouble starting the conversation, they stumbled over the first words.