Short answers, pulled from the story.
Blowback refers to the unintended consequences and unwanted side effects of covert operations. The CIA coined the term to describe harm that falls on friendly populations and forces when a secret operation produces results the sponsoring party did not intend. To civilians, the resulting violence often appears random because the secret attacks that provoked it are unknown to the public.
The term blowback first appeared in print in a CIA internal history titled Clandestine Service History: Overthrow of Premier Mossadeq of Iran, covering November 1952 through August 1953, published in March 1954. That document recorded the American and British-sponsored coup that removed Iran's prime minister.
Secret US funding for the Contras, aimed at fighting Nicaragua's Sandinista government, led the Reagan Administration to sell weapons to Iran to keep the Contras supplied, which became the Iran-Contra Affair. The financial pressure on the Contras also pushed them into drug-dealing in American cities. The International Court of Justice later ruled against the United States in Nicaragua v. United States over the secret military attacks.
Osama bin Laden is cited as an example of blowback arising from CIA financing and support for Afghan insurgents fighting the Soviet Union in Afghanistan. He was among the anti-Western religious figures who received covert support and later attacked both his adversary and his original sponsor.
Russian military intelligence recruited volunteers including Shamil Basayev, Ruslan Gelayev, and Umalt Deshayev to fight alongside Abkhaz separatists in the 1992-1993 war. After that conflict ended, many of the same men fought against Russia in the First Chechen War, leading units known as Abkhaz battalions. All three commanders who emerged from Russia's original volunteer network were ultimately killed by Russia itself.
Yevno Azef was a Russian socialist revolutionary who also worked as a paid informant for the Okhrana, the imperial Russian secret police, earning 1,000 rubles a month by 1908. He used his police-facilitated rise within the Socialist Revolutionary Party to organise the assassinations of Interior Minister Vyacheslav Plehve in 1904 and Grand Duke Sergei Alexandrovich in 1905, turning the intelligence investment into lethal violence against the sponsoring state.