Whilst trying to fix #2411 (Validation errors in template filters/tags documentation) I came across this minor issue. The templates loop over each tag/filter, and title them with something like <h3 id="{{ tag.name|escape }}">{{ tag.name|escape }}</h3>.
When there are two template tags named the same, then there will be an id clash, causing an (x)html validation error (and the anchor tags won't work properly for the two duplicate id's):
# Error Line 525 column 12: ID "filter" already defined.
<h3 id="filter">filter</h3>
An "id" is a unique identifier. Each time this attribute is used in a document it must have a different value. If you are using this attribute as a hook for style sheets it may be more appropriate to use classes (which group elements) than id (which are used to identify exactly one element).
Currently, this is only an issue in template_tag_index.html, where we have a filter tag in the defaulttags section and in the admin_list section.
It can be fixed by prepending the library name to the id, so that each one will be unique. I'll add a patch once I've finished up #2411
