Final answer:
Snape threw cockroach clusters at Harry after catching him viewing Snape's personal childhood memories in the Pensieve.
Step-by-step explanation:
Professor Severus Snape threw a jar of cockroach clusters at Harry Potter when he caught him viewing the Pensieve, which contained Snape's old childhood memories. This incident occurs in 'Harry Potter and the Order of the Phoenix' during a moment when Harry's curiosity got the better of him, and he inadvertently invaded Snape's privacy. Snape's fury was intense as these memories were deeply personal and a source of pain and humiliation for him.