The built-in flash automatically fires even without being previously set. (EOS REBEL T3i/EOS 600D)
09-Feb-2011
8201006600
Solution
In the < 
>(Scene Intelligent Auto), < 
>(Portrait), or < 
>(Close-up)mode, the camera automatically judges the shooting environment to determine whether the built-in flash fires or not. Therefore, the built-in flash may automatically fire in low-light environments even if you do not want to use it.
>(Scene Intelligent Auto), < >(Portrait), or < >(Close-up)mode, the camera automatically judges the shooting environment to determine whether the built-in flash fires or not. Therefore, the built-in flash may automatically fire in low-light environments even if you do not want to use it.To prevent the built-in flash from firing automatically, change the shooting mode.
For the details, see the built-in flash section at List of functions that can be set in each shooting mode (EOS REBEL T3i/EOS 600D).