Only happened on ssa/ass subtitles, and often happened on those translated subtitles like Chinese subtitle of English movies. I guess the translator amended the original subtitles by adding missing words, notes, and translator names.
Here are 2 samples:
http://www.zimuku.net/download/ODU0ODN8MDdlOTliMGE0ODM4YzlmYmQ1MGQ4Yjk5fDE0OTEzNjY0NjB8YjViOWFlYmZ8cmVtb3Rl
The amended subtitles are added at bottom:
Dialogue: 0,2:05:44.76,2:05:55.29,Default,0000,0000,0000,(end of movie time frame)
Dialogue: 0,0:03:39.12,0:03:49.09,Default,0000,0000,0000,some notes
Dialogue: 0,0:03:49.12,0:03:59.09,Default,0000,0000,0000,some notes
…
http://www.zimuku.net/download/Njg1NzB8YjI0ZjRhZTJlYjQxYTU3M2U5ZDIxZDcwfDE0OTEzNjY2Nzd8ZWIxMGU5OTB8cmVtb3Rl
Check the Chs&Eng subtitle in the rar file, they added on top:
Dialogue: Marked=0,0:00:10.46,0:00:15.39,Default,NTP,0000,0000,0000,some notes
Dialogue: Marked=0,0:00:10.46,0:00:15.39,Default,NTP,0000,0000,0000,some notes
Dialogue: Marked=0,0:00:17.50,0:00:26.00,BGN,NTP,0000,0000,0000,Scroll up;360;100;40;[20],some notes
Dialogue: Marked=0,0:00:22.00,0:00:29.50,BGN,NTP,0000,0000,0000,Scroll up;360;100;35;[20],some notes
Dialogue: Marked=0,0:00:23.50,0:00:31.00,BGN,NTP,0000,0000,0000,Scroll up;360;100;35;[20],some notes
Dialogue: Marked=0,0:01:16.00,0:01:24.00,BGN,NTP,0000,0000,0000,Scroll up;360;100;50;[10],some notes
Dialogue: Marked=0,0:01:18.00,0:01:26.00,BGN,NTP,0000,0000,0000,Scroll up;360;100;50;[10],some notes
Dialogue: Marked=0,0:01:20.00,0:01:28.00,BGN,NTP,0000,0000,0000,Scroll up;360;100;50;[10],some notes
Dialogue: Marked=0,0:01:22.00,0:01:30.00,BGN,NTP,0000,0000,0000,Scroll up;360;100;50;[10],some notes
Dialogue: Marked=0,0:01:24.50,0:01:36.00,BGN,NTP,0000,0000,0000,Scroll up;360;60;50;[10],some notes
Dialogue: Marked=0,0:01:26.50,0:01:37.00,BGN,NTP,0000,0000,0000,Scroll up;360;60;50;[10],some notes
Dialogue: Marked=0,0:01:28.50,0:01:38.00,BGN,NTP,0000,0000,0000,Scroll up;360;60;50;[10],some notes
Dialogue: Marked=0,0:01:31.00,0:01:43.00,BGN,NTP,0000,0000,0000,Scroll up;360;30;50;[10],some notes
Dialogue: Marked=0,0:01:33.00,0:01:44.00,BGN,NTP,0000,0000,0000,Scroll up;360;30;50;[10],some notes
Dialogue: Marked=0,0:01:35.00,0:01:45.00,BGN,NTP,0000,0000,0000,Scroll up;360;30;50;[10],some notes
Dialogue: Marked=0,2:07:07.53,2:07:09.49,Default,NTP,0000,0000,0000,some notes
Dialogue: Marked=0,2:07:09.94,2:07:12.76,Default,NTP,0000,0000,0000,some notes
Dialogue: Marked=0,2:07:13.44,2:07:15.49,Default,NTP,0000,0000,0000,some notes
Dialogue: Marked=0,2:07:16.35,2:07:19.25,Default,NTP,0000,0000,0000,some notes
Dialogue: Marked=0,0:00:00.30,0:00:04.00,Default,NTP,0000,0000,0000,!Effect,(movie time frame starts here)
…