YouTube has a feature: YouTube Topics. According to YouTube, "YouTube Topics is a new way to explore the worlds of videos on YouTube. After you opt in, when you search for something ("funny" for example) you will see topics related to your current search displayed at the top of search results and next to individual videos.". To opt-in, visit this link.
Once you opt-in, search for the query "mashup". You will see a special notification saying: "You found a new golden topic! You’ve found 1 of 17.". It means that there are 17 golden topics, and you have got one. If you can find all of them, you’ll unlock a special YouTube logo to prove your puzzle prowess.

Thankfully, you don’t need to guess rest of the golden topics. Here is the list of all seventeen golden topics:
- high speed camera
- cup stacking
- cheese rolling
- dancing baby
- double rainbow
- urban exploration
- funny cats
- speed flying
- optical illusions
- ferrofluid
- ukulele
- nursery rhymes
- how it should have ended
- mashup
- gingers
- scary maze
- tilt shift
After completing search for above 17 keywords, you will unlock above golden YouTube logo. Well, this logo will disappear after clearing browser cookies. If you want to place the golden logo permanently, even without search above golden topics, install this greasemonkey script (works fine in Chrome and Firefox). This may be a time waster, but if you love discovering easter eggs, you will certainly like it.
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Wow! Thanks for sharing :P