This is a real edge-case but...
When we add a tag to a tiddler, a place-holder tiddler gets created for the tag itself. I wanted to organise these tag tiddlers by tagging them, so I made a little do-dah to do it.
<table>
<$list filter="[!is[system]tags[]]">
<tr>
<td><$edit-text field="title"/></td>
<td><$checkbox tag="Language"/></td>
<td><$checkbox tag="Platform"/></td>
<td><$checkbox tag="Technology"/></td>
<td><$edit-text field="tags"/></td><td><$link>{{!!tags}}</$link></td>
</tr>
</$list>
If I click the checkboxes, it adds the respective tag to the tiddler and then it exists as a real tiddler (not just a placeholder) BUT it doesn't get a 'created' field, only a 'modified' field.
This is 'unexpected behaviour' because the tiddler exists but was never created. I only noticed this because I was subsequently trying to list all my tiddlers chronologically by creation date.
Sadly I'm not yet at the stage of understanding the code well enough to propose a fix, but I suppose that when a tiddler is modified we might check that it already has a created field and, if not, add one. Alternatively, perhaps there is a good reason for this behaviour?
As I say.... just an edge case :)
Regards,
Richard