Assume that you composed a song titled "I have a Copyright". If an infringer copied your song or distributed it on a music sharing site, it may be very easy or extremely difficult to locate these files.
If you use a search engine (i.e. Google) to enter the keyword "I have a Copyright", the results will include websites where an exact copy is located. However, if they changed the title to "Kopyright is mine", the search engine may not find anything.
What if the infringer changed the title to "Cats and Dogs"? It will be virtually impossible to locate infringement unless you were to listen to every music file on the Internet. However, by the time you finished listening to first 1,000 songs, they could have uploaded the file to a site you already checked.
What if somebody copies a photograph or artwork? It would be easy to locate a photograph titled "Likeness of a Copyright" when the infringer uses the same title. However, what happens when the infringer copies your photograph of a rainbow over a bridge and calls it "Clouds reflected on the water"? Keywords will not locate the infringement. In this scenario, you would have to do a brute force search of photographs or paintings and hope you get lucky.
Assume you wrote a poem. An identical copy would be easy to locate, since the verses were not changed. However, what if the infringer copies only a portion of the poem and uses it within an otherwise original composition? A search would include each verse of your poem to find a comparison. Even if it were an exact copy, you would still have to search every web page on the Internet and greeting card in the world.
Recent Examples in the News
The U.S. Postal Service was ordered to pay $3.5 million in damages for mistakenly using a Las Vegas statute on a postage stamp design instead of using the real statute in New York. The postal service had obtained the photo from Getty Images and thought it was the real statute.
The song "Happy Birthday to You" involved a copyright that was recently ruled invalid. As a result, Warner Music Group had to pay $14 million in claims. This involved licensing fees that had been paid for a song that was actually in public domain.
Zillow able to reverse an infringement claim regarding photos on its real estate website. A photography studio had sued Zillow for using photos on its listing platform.