Bump into
Definition 1 of 2
Pro Tip 1/3Chance Meeting
Use 'bump into' when you unexpectedly meet someone you know. 
Guess who I bumped into at the concert last night? My old roommate!
Pro Tip 2/3No Prior Plan
It emphasizes a meeting was not planned beforehand. 
I bumped into my teacher while shopping, what a surprise!
Pro Tip 3/3Small Talk Trigger
Often leads to a brief and casual conversation. 
When I bumped into Jane, we chatted for a few minutes.