coming across/running into a black cat

 Hi,

Regarding unexpectedness, I find come across and run into do have some similarity. Both can be used with either something or somebody. For examples:

 
1a. I came across an old building.

1b. I came across an old diary in her desk

1c. I came across this problem.

1d. I ran into some trouble.

1e. He ran into criticism after remarks he made in a television interview.

2a. I had come across so many foreigners who have told me tales of the wonders of their own land.

2b. I've never come across anyone quite like her before.

2c. I came across / ran into a herd of sheep.

2d. I came across / ran into an old friend of mine.

In short, I'm still at loss for the exact rules that dictate the difference between the two phrasal verbs. Nevertheless, I sense that people don't use run into with either someone they meet the first time or insentient (physical) objects.

By the way, somehow I feel come across does not fit well with either 1d or 1e, but I cannot explain why.

Well,  I will say this.  Wth either expression, I am sure people won’t question you what you exactly mean when you say “I came across / ran into something really spooky in the library when I was doing some studying late last night, I felt something across the table when there was no one there”.  Here, the context clearly suggested that something unexpected happened in your presence, and both expressions can comfortably fit into this context.

I think we are trying to whip up scenarios which either disprove or agree with the use of either one.  In my previous post, I expressed my scenario and I am sure some may be thinking hard to come up with a differring scenario.

Certain scenario with words such as “problems” and “situations” is more commonly tied to the expression “run into”.

“Come across/ upon” = find unexpectedly, as in "the archeologists chanced upon an old tomb"; "she struck a goldmine";  

In either case, they both contain a sense of unexpectancy. Did I just ^{stumble on} something!