Wikipedia:Don't throw more litter onto the pile

Picture this. You're walking along a path in the countryside, and you've just finished that bottle of water. The problem is, there're no bins nearby. As you turn a corner, you notice a pile of rubbish on the floor. There's all sorts of garbage here: old boxes, tyres, food wrappers, and even empty bottles. It's a pretty big pile, and you've still got that empty water bottle in your hand with no bins in sight.

You think to yourself, "well, people have already littered here – what's one more? I guess it's okay to do it in this one spot." You toss your water bottle down on the pile, stride away, and are promptly struck by lightning.

Sourcing is important
Adding unsourced items to an article with the justification that there's already unsourced content is no different to the littering analogy above. The goal on Wikipedia is to improve the encyclopedia, not make it worse.

Content on Wikipedia needs to be verifiable and reliably sourced. Wikipedia isn't perfect and is a perpetual work in progress, which means there will often be content in an article that should be sourced but isn't. This doesn't mean it's acceptable, it just means that it has yet to be sourced or removed. Adding to it makes the situation worse; instead good editors should lead the way in adding content that is backed by reliable sources.

Adding litter to an existing pile is particularly pronounced on lists articles, or articles that contain lists of examples. Users over the course of months or years might gradually expand the list, but often by adding unsourced material. If you come across a list and feel you could add something to it, the lack of sourcing for other items doesn't excuse the addition of more unsourced content. Indeed, the existence of such a list suggests often calls for removing content or adding sources.