I was at united.com looking for fares from WAS to LAX for this weekend. Saw $382 or so r.t. 28th-30th, $256 for 27-28th, and $318 for 28-29th which are the dates I want just they don't have a redeye that night.
So, I went back and checked 28th-30th. First thing I noticed was that the routing offered was now connecting flights instead of the nonstops. Next thing of notice is the fare of $298. Checking the 28th-29th and see the same fare for a (connecting) redeye coming back.

This all happened in about 5 minutes (maybe a bit more) around 11:45AM.

Yes, I noticed something very similar recently. Apparently the technology now is such that you are looking at live fares and routings that can literally change before your eyes.

I am sure it happens. But this was a false alarm. I had changed the month of the return to be December! Thinking about nesting (?) two trips.

I recently had a United fare drop by $20 in between me selecting the flight and then actually booking it. Much better than having it go up!

Many websites have that capability now, and it can go both ways, with fares rising or falling. Chances are that someone else bought a ticket on the flight and affected the price for you while you were searching