For example, user contributions, page history, logging etc. These pages use the last timestamp returned and on the next page it uses WHERE xx_timestamp < $offset, so if not all values having the same timestamp were returned on the previous page, they're missed in paging. This is just bug 24782 applied on non-API pages.
See Also:
- T26782: API uses non-unique value for paging for some modules (API)
- T4930: Edit history is based on timestamp while diff prev/next links are based on revision id
- T44296: page history shows unreviewed revisions, reverts in wrong order
- T46841: Contributions appearing out of order
- T91883: prop=revisions sorts by rev_id, not by rev_timestamp
- T114756: Special:Contributions should sort by rev_id if two revisions have the same rev_timestamp