Accidentally stopping malware

The researcher first noticed that the malware was trying to contact a specific web address every time it infected a new computer. But the web address it was trying to contact - iuqerfsodp9ifjaposdfjhgosurijfaewrwergwea.com - had not been registered. MalwareTech decided to register it, and bought it for $10.69 (£8). Owning it would let him see where computers were accessing it from, and give him an idea of how widespread the ransomware was.

But by doing so he triggered part of the ransomware's code that told it to continuing spreading as long as the website iuqerfsodp9ifjaposdfjhgosurijfaewrwergwea.com did not exist. This type of code is known as a "kill switch", which some attackers use to halt the spread of their software if things get out of hand.
http://www.bbc.com/news/technology-39907049