Death of Leslie Shay
In reference to the episode: "Always".
Shay was killed in the line of duty while attending a a building fire with Truck 81, Squad 3 and Ambulance 61. It is later found out that the fire was a trap, set by a serial arsonist for the first responders.
After everyone had entered the building, and Shay and Dawson began treating the victim, the building exploded; leaving Peter Mills with a broken leg, however everyone else seemed to be okay.
However, the explosion led to a metal pole hitting Shay on the head, killing her instantly, despite resuscitation attempts by both Dawson and Severide.
Dawson feels extremely guilty about Shay's death, not only because they were best friends, but that Shay swapped places with her because Dawson was going to transfer to be a firefighter - if they hadn't switched places, it would have been Dawson hit by that pole, not Shay.