All the sins of my followers will be forgiven except those

The Prophet (S.A.W) said that: 

"All the sins of my followers will be forgiven except those

 of the Mujahirin (those who commit a sin openly or disclose their sins to the people). 

An example of such disclosure is that a person commits a sin

 at night and though Allah screens it from the public, 

then he comes in the morning, and says,

 'O so-and-so, I did such-and-such (evil) deed yesterday,'

 though he spent his night screened by his Lord (none knowing about his sin) and in the morning he removes Allah's screen from himself." 

[Bukhari: Book 8 Volume 73 Hadith 95]