Why can't I start the tags with a dot?

If you start the tags for a question on this site with a dot e.g. “.doc” the question is not posted without any feedback, while the “.doc” tag is suggested in the drop down list box with tags.

Please retag your question to meta and remove common (common is for problems present in all LO apps).

I can’t answer your question. I suppose at some time the faulty tag was entered and remembered by the engine though it is rejected by the tagging function. This site suffers from many ergonomics shortcomings. This is probably yet another one.

There are some feedback, but it is no clear:

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Just retaged to meta. Try with .doc if you want to get the red sign.