Difference between "run into", "come upon" and "come across"
Solution 1:
In my experinece you run into people and trouble:
I ran into Betty at the store today. I haven't seen her since college!
I ran into your co-worker Bob today.
Sorry I'm late; I ran into heavy traffic.
You come across objects:
I was cleaning up the kitchen and came across that phone bill you lost.
I was looking up (something) in the encylopedia and came across this interesting tidbit.
I'm not sure I'd use either when talking about a grammar error in a blog; I'd be more likely to say I found or noticed it. I don't know why that is.
Solution 2:
When you run into something, it's likely to be metaphorical and those metaphorical encounters are usually negative. The most common noun collocates are: problem, trouble, wall, difficulty, opposition, resistance, and roadblock. It also commonly means meeting a person unexpectedly.
The next item, come upon, is far less common than run into and is usually used when have physically moved into a situation or location. The most common noun collocates are: scene, sight, and accident. And the most common adverbs are suddenly and finally, which are almost equally frequent, suggesting this is used both when you're searching and when you're not.
About as frequent as come upon is come across, the most common meaning of which is that you've physically travelled across something (e.g., a bridge), and there's also the meaning of how you are perceived (i.e., come across as), but in the sense suggested by this group of finding vocabulary, usually it's something your find while reading. Relevant collocations are: article, reference, and script.
Solution 3:
They all mean "to encounter unexpectedly", but are used in different contexts:
Run into means to encounter someone (or something) during your normal course of events, often while physically traveling: I ran into Sally at the supermarket.
Come across means to find something (or someone) when you were not looking for him/her/it: I was cleaning out the garage when I came across my high school yearbook.
Note that the phrase "come across" has a completely different sense where it also means "to be understood (perhaps wrongly)": I tried to tell her I was worried about her, but I'm afraid I came across as critical instead of concerned.
- Come upon means to find a thing or (often) a situation during your course of events, somewhat similar to "run into" except usually referring to inanimate things or situations and without the implicit subcontext of physical travel: While reading Paradise Lost , I came upon the most amazing literary passage. It's often used interchangeably with "come across" (the first sense).
I disagree that "come upon" is now uncommon. It's never been an overly common expression, but I think it's probably used as much now as ever.
Solution 4:
The distinctions are very subtle, and while I don't necessarily disagree with Brett, I think he's overstating. To really answer this you might need to study many examples in context.
I think that "run into" tends to be used to describe something you find accidentally when travelling. (Not necessarily to a foreign country or anything like that, it could be across the street.) "Come upon" and "come across" tend to be used when describing something you find while searching for something else or working on something else.
Like, "I was in the grocery store yesterday when I ran into my friend Sally." But, "I was searching the closet for my brown shoes when I came across some old photographs." You wouldn't normally switch the two. But the differences are subtle and if you did, it would sound only slightly odd.
Of course if you say, "I was driving down the street at 60 mile per hour when I ran into Sally", that means something quite different.
Solution 5:
Come across something
Run into somebody