List of Reasons for Automatic System Content Modification

Question

When does the system automatically modify content (such as votes, question, answer, or comment text)?

What automatic changes does the system make to

• questions?
• answers?
• comments?

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This FAQ should help new users understand potentially surprising behavior to avoid erroneous bug reports and recurring questions such as new users are asking why their posts don't come out the way they typed them

Answer 1

Questions

• If a question is closed as a duplicate, its title will be rendered as Original Title [duplicate].
• If a question is closed (not as a duplicate) within the last five days, its title will be rendered as Original Title [closed].
• If a question is migrated to another site, its title will be rendered as Original Title [migrated]
• If a question was migrated from another site and subsequently closed for any reason, its title will be rendered as Original Title [migrated] in search results only.

Note that the above rules only apply to the website (including the mobile site), not the mobile apps. On the mobile apps, no title modifications are performed when viewing the question, only in question lists. There, the first three above rules apply, in addition to the following ones:

• If the question is protected, its title will be rendered as Original Title [protected].
• If the question is locked, its title will be rendered as Original Title [locked].

Additionally, in both the app and the website:

• Tags using [tag] notation are automatically stripped from titles.
• Exclamation marks at the end of titles are converted into question marks.
• "Straight quotes" in question titles are converted into “smart quotes”.
• The sequence "--" is converted into an em dash ("—") in question titles. To force it to display as "--", put an invisible character in between the two dashes, like in the following sequence: -​-

Answers

• Short answers with links internal to the site are converted to comments.¹

All Posts

• Greetings are removed automatically from both questions and answers before they are posted.
• Internal links to questions are converted into links as the title of the question. For example https://meta.stackexchange.com/questions/97524/list-of-reasons-for-automatic-system-content-modification gets converted into List of Reasons for Automatic System Content Modification.

Comments

• In comments containing @replies to the post owner, the @reply will be removed from the comment text if it is at the beginning of the comment and the author has not commented, as it is not needed (the post owner is always notified of comments on their post).
• When a question is closed as a duplicate, the automatically generated "Does this answer your question?" comment posted at the time a vote or recommend closure flag was cast will be automatically deleted if the target it links to is selected as one of the duplicate targets, provided it's not been edited.

Information above is taken from: Recent feature changes to Stack Exchange.
¹ This applies even if the user posting the answer doesn't have enough reputation to post comments.