The Baader-Meinhoff Effect
So you just watched a video on Agaricus bitorquis, or the sidewalk mushroom. The next day, you spot one on your way to college. A couple days later, you come across a recipe book in which you find a recipe that claims to be the best sidewalk mushroom soup in the world. Fast forward another couple days - you're turning the corner in your street, and you witness the indomitable fungal spirit in the form of an Agaricus bitorquis sprouting from the asphalt.
No, the universe is not out to get you. This is simply a cognitive phenomenon called the Baader-Meinhof effect. Contrary to what you might have deduced, Baader and Meinhoff are not the science people who discovered this effect. The Baader-Meinhof Gang was a left-wing extremist group active in 1970s Germany. The naming of the phenomenon is generally credited to Terry Mullen, who wrote to a Minnesota newspaper in 1994, about an incident related to it. Mullen coined the term due to the experience of having discussed the Baader-Meinhof Gang and then witnessing the name in the next day's newspaper, despite the fact that the group hadn’t been newsworthy for many many years.
More scientific terms for this phenomenon would be "frequency illusion", "frequency bias", or "selective attention bias". You notice something for the first time- like a species of mushroom- and you end up noticing it around you more often than you ever have. This leads you to believe that there's an increased frequency of its occurrence after you learned about it. Thus it's also called "recency illusion".
It's pretty simple as to why it happens - your brain filters out many things happening around you and selectively concentrates on things that are perceived as necessary, or interesting. This makes you perceive it consciously later on.
The frequency bias is notorious for tampering with the legal system, as both detectives and eyewitnesses are at the end of the day people fallible to their cognitive biases. Your biases simply cannot be trusted as concrete evidence.
Even researchers are fallible to this illusion, due to which scientific research could be disrupted. Advertisers exploit this effect for profits. But that's on capitalism, not your brains.
Frequency bias is not a scary monster you have to be wary of. It's not a bug, but a feature. It has helped our species to commit completely new concepts to our brains, and reinforce that information to help us evolve and adapt to even the most dangerous of circumstances.