Usually it just the tile slide and leave opening gap if you talk about common tile roof.

I just fix mine when the one at the edge almost fell off. I just paid the guy RM80. My roof is very sloppy type. I help him to hold the ladder on the lower level roof. He found another one sliding too. To prevent future slide, I bought silicone glue to fix. The grey colored one will stay flexible for probably 20 years.