The Danger Room is capable of generating realistic 3D images of locations and people.
The former X-Man, Morph, who at the time was under the control of Mister Sinister, once broke into the mansion and turned up the danger level in the Danger Room while Beast was practicing in it. This almost killed Beast. However, he was able to survive.
When Spider-Man broke into the Xavier Mansion to see Professor X, the X-Men chased him down. Spider-Man attempted to get away from the X-Men and accidentally ran into the Danger Room. Gambit revealed to the other X-Men that earlier he was training in the Danger Room and accidentally left it it running. When Cyclops asked what computer program he was using Gambit answered "Deadly Sentinels". Spider-Man then fought holographic images of some Sentinels. The x-Men broke into the Danger Room and helped Spider-Man defeat the Sentinels. While the X-Men did this Jean Grey ran to the control room and shut down the Danger Room.
When Juggernaut broke into the X-Mansion to kill Professor X he ended up in the Danger Room. Professor X then activated the Danger Room and Juggernaut fought robotic duplicates of himself and a robotic duplicate of the Hulk.
In the comics
When the Danger Room developed artificial intelligence it uploaded itself to the body of an old Sentinel and took the name Danger.
Danger then manipulated one of Charles Xavier's students, Wing, into committing suicide. Wing's death voided the Danger Room's safeguards allowing Danger to pursue it's mission of destroying the X-Men.
Possessing complete knowledge of the X-Men's fighting techniques and weaknesses Danger was easily able to defeat the X-Men.
Danger then traveled to Genosha to kill Professor X. Initially defeated by Xavier, Danger uploaded it's programming into a Sentinel. After the X-Men arrived Shadowcat phased inside the Sentinel and reactivated it's memories of the destruction that Danger had repressed. Overriding Danger's consciousness, the Sentinel became horrified at what it had done and left.