come across (something)

This phrase means to find something without really trying to look for it:

But sometimes you'll be flipping through the channels and come across something really interesting.

Aren't these great? I came across them at a thrift shop a couple weeks ago.

Sometimes (someone) will be (doing something) and (something will happen)

This phrase describes a situation that sometimes happens. It uses the future tense "will", but it doesn't describe something that happens in the future. Instead, it's something that has happened in the past several times and might happen again. Here are some other examples:

Sometimes I'll be sitting in my office and Nadine will yell out a question to me from the office next door.

You know how sometimes you'll be driving along and another car will pull out in front of you all of a sudden? That makes me so annoyed.

flip through (something)

To "flip through the channels" on your TV means to change the channel again and again to search for something you want to watch. You can also "flip through" other things:

• the pages of a book
• CDs in a music store

something (adjective)

Call an interesting thing "something interesting". Call a funny thing "something funny". Call a frightening thing "something scary":

I heard something scary on the news this morning.

"Something ___" is more natural than saying "a ___ thing".