[pests]

There are some auto rewrite rules. Off the top of my head: numbers in the beginning are stripped, [pdf] or [video] can be added to the end, and one more I can't remember that gets stripped off beginning and can cause confusion.

A pdf link to "5 Reasons To Do Things" will be "Reasons To Do Things [pdf]" for example.

[Tomte]

„How“ at the beginning is stripped, leading to all these strange sounding „I <verb>“ submissions.

[TRiG_Ireland]

There was also an interesting article on assistive technology called "How Disabled People Use the Web", or something similar, which looked very silly with the "How" stripped.