Inspired by this question on meta.cstheory.SE. The post at meta.Tex.SE is quite impressive.

We often leave generic comments to OP and answer posters such as: "if this is a homework, please add a tag," and such. Can we make this post a community wiki and add a big list of standard comments? Let's annotate the usage cases, so other users and copy-and-paste whenever applies.

As usual on meta, (down/up)vote if you (dis/)agree.

Update: To facilitate copy & paste, if your comment contains links and/or formatting, please enclose the whole text between single inverted quotes: `...` i.e. verbatim like this.