I opened an older world of mine (1.7.9) because my house was set up on the edge of a Deep Ocean Biome and I had been told that Ocean Monuments would spawn in older worlds. When I got in, I immediately found Guardians right next to my house, but there was no Ocean Monument anywhere. Only Guardians spawned, not Elder Guardians.

What should have happened: Guardians should have spawned around an Ocean Monument, or not at all.

What did happen: No Ocean Monument, but Guardians being their normal annoying selves all the same.