When it comes to movies, I always research them before I watch them these days. I had a really unpleasant experience that way once which affected me for weeks.
Some people don't like the use of trigger warnings and feel like the anticipation does more harm than good. I personally like them and haven't seen people go overboard with them. However, I only use WP, so I might not be the best person to ask.
It's impossible to know what will or won't trigger someone, but there are a handful of topics that are often triggering for those who have PTSD or who have experienced trauma. Having a warning for those specific topics, especially in threads where a person might not be expecting to see a trigger, is considerate IMO. Depending on the specific thread, a content warning in the title could be helpful too. Some topics that I think are triggering for a lot of folks are mentions of sexual abuse, torture, eating disorders, and suicide.
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Better to reign in Hell than serve in Heaven. – Satan and TwilightPrincess