During Harry's Occlumency lessons with Snape in the Order of the Phoenix, Harry uses Protego in response to Snape's attempt to enter his mind. This results in the spell rebounding and consequently Harry sees some of Snape's memories.
Quote from Harry Potter and the Order of the Phoenix C.26 P.591-2
Harry raised his own wand.
Snape staggered; his wand flew upward, away from Harry — and suddenly Harry’s mind was teeming with memories that were not his — a hook-nosed man was shouting at a cowering woman, while a small dark-haired boy cried in a corner. . . . A greasy-haired teenager sat alone in a dark bedroom, pointing his wand at the ceiling, shooting down flies. . . . A girl was laughing as a scrawny boy tried to mount a bucking broomstick —
Harry felt as though he had been pushed hard in the chest; he took several staggering steps backward, hit some of the shelves covering Snape’s walls and heard something crack. Snape was shaking slightly, very white in the face.
Considering that Harry was new to Occlumency at this point and was struggling to grasp it, how could he have the ability to get into Snape's mind, even with the Shield Charm? Surely Snape would be strong and experienced enough to withstand this?
In summary, did Snape allow Harry to see his memory during an Occlumency lesson?