Varies by story but...
Dane Coleman gets a job to support his mother and sister instead of joining the Marines. Never develops he skills needed, and either the Resistance selects someone else to stop the bad guy or he just wins eventually (Digital Cowboy Dane on Pocket FM)
Kelly Pierce decided he wants to be the one who lives, instead of the unknown woman in another universe. An entire world is lost eventually, maybe more (Strange Awakening).
David West decides not to overspend at OmegaCon, gets a cab ride home, and never meets Thellissandra, remaining in the dark about his own heritage, probably remaining single and never giving birth to the children that will potentially save all of reality (Between Earth and Pyrroth on Pocket FM).
Jack Diamond follows orders, Carol is sacrificed, Morgalis summons Tennigrath Lord of the Void, and the world's extranormals (superheroes) are forced into an epic confrontation that many fail to survive. A major city is likely devastated as a result, with massive loss of lives. Jack survives, but dies of cirrhosis of the liver from excessive drinking, before lung cancer would get its shot at him (Diamond in the Rough, Blood Diamond).
Indigo picks a different person's life to step into after their death instead of a future cop - could go either way for him, nothing changes for the world overall, but his child's life may be very different. His life won't be any better, but it might be longer (True Blue).
Nicholas Sato decides to stay with his school group instead of exploring the mysterious staircase he accidentally discovers. He dies in the kaiju attack on the factory, the M-78 is buried in rubble and never rediscovered, so the Exerians conquer the Earth and continue their swathe of conquest across the universe until a new challenger appears somewhere (The Kaiju System on HoneyFeed).