The classic example is a bench with armrests added to it later, to stop homeless people from sleeping on it. Another is obstacles added to a railing or curb to stop skateboarders from using it "incorrectly".

Please note that "I think this is a good idea actually" doesn't mean it's not hostile architecture, if it reasonably fits the definition above.

More information is available on Wikipedia

Rules

1) Scope of the subreddit

Submissions must show hostile intent, and not poor design. If it doesn’t directly inconvenience people, it is a better fit for /r/crappyarchitecture.

On some occasions, hostile architecture could be accidental, but have the same result. If it's interesting, it's allowed.

2) Treat all participants with respect

Please be civil in comments and posts, and treat all participants with respect. No low-quality anti-homeless sentiment, e.g. "hurr durr hobo bad," please. Thoughtful discussion on the issue of homelessness and hostile architecture in relation to homelessness is permitted and welcomed, but disrespectful comments towards people experiencing homelessness is not allowed.

3) No advocating for destruction of property

No posts or comments advocating destruction of property, vandalism, or other illegal activity.

4) Flair your post

Please assign the flair that best fits the subject of your post.

5) No anti-homeless sentiment

Homeless people are valid users of public spaces, so they can be the subject of hostile architecture. If generic complaints or insults about the homeless are all you have to contribute, you will be escorted off the property. If you have specific information regarding a post, respectful discussion is welcome.