sticky fingaz got mad at 50 over a line in "how to rob*" and dissed him in "jackin 4 beats."
jay-z got mad over a line in "how to rob" and retaliated at a summer jam, then 50 rebutted with "be a gentleman."
the ja rule got mad because 50's boy took ja's chain in queens. (more details in article: Article pt. 1*
Article pt. 2*

50 Cent Lyrics.

that should be just about all you need to know.
*articles found in 9-01 issue of XXL.

edit - had to fix urls.