There are my older “poltergeist” adventures and particularly this post of yours How to snatch tags from another tiddler? - #20 by EricShulman , which recommends more caution about using particularly = and generally any special symbol as tag prefix.
There are new TiddlyWiki users tempted to use # as tag prefix a la Twitter hashtags, while # is at least used in HTML anchors.
There’s the Namespacing section in Grok TiddlyWiki — Build a deep, lasting understanding of TiddlyWiki , which may make it tempting for some users to use just : as prefix for personal tags… and then they learn that : is a legit citizen in filter syntax too, albeit not as often as [. The problem is that a subset of new users (myself included) start with the official TiddlyWiki docs as primary knowledge source, and only later add GrokTiddlyWiki as secondary, complementary wisdom resource. Also, most if not all new users start making use of tags long before they start writing wikitext filters, thus learn about the special meanings of special symbols in filter syntax. The warning for special symbols in tiddler titles that @pmario mentions above, is relevant (since tags are tiddlers), yet very succint.
These facts only prove that tag naming conventions are a big and complex issue, and that new users inevitably end going through changing their tag naming policies and painful refactoring of their wikis as they learn more.
Perhaps they can be helped by having a set of postulates explicitly carved in stone in the official docs, like having a table with all these special symbols and for each of them a couple of examples when using them as tag prefixes causes troubles, similar to the “pesky brackets”™ resource? Something like:
[ - widely used in filter syntax, also clashes with “no nested brackets” rule
+ - used as filter run prefix
: - used as filter run prefix, operator suffix
etc