As part of their training, firefighters are taught about flashovers. This is when a majority of the combustible materials in a room reach the autoignition point and burst into flames. This immediate conflagration can trap people in a room with no avenue of escape.
There are warning signs that firefighters are trained to look for that rely on being pro-active rather than reactive. If extreme fire behavior happens and a firefighter misses the signs, it may be too late to get out.
Firefighters go into burning buildings every day, knowing they may not be able to read the signs. Indicators of a flashover may be seen, heard, or felt, and firefighters are encouraged to get out of a room immediately if something doesn't feel right: "obey the instinct".
In Staten Island, NY a fireman faced this exact phenomenon while searching for victims. He became trapped in an apartment room because the conflagration sprang up and blocked the door. Despite suffering several burns, his training kicked in and he was able to breach a wall and escape.
This situation highlights why communities need highly trained professionals to answer the call, night or day. They do it so we don't have to!