I encountered a bug when I wanted to embed metadata.
Spoiler:
calibre, version 3.46.0
WARNING: Failed for some files: Failed to embed metadata into some book files. Click "Show details" for details.
The EPUB format of Alexandra-Ivy-Gardiens-de-léternité-T8.5-Luc:
Traceback (most recent call last):
File "site-packages\calibre\customize\ui.py", line 447, in set_file_type_metadata
File "site-packages\calibre\customize\builtins.py", line 467, in set_metadata
File "site-packages\calibre\ebooks\metadata\epub.py", line 301, in set_metadata
File "site-packages\calibre\ebooks\metadata\opf.py", line 113, in set_metadata
File "site-packages\calibre\ebooks\metadata\opf.py", line 100, in set_metadata_opf3
File "site-packages\calibre\ebooks\metadata\opf3.py", line 1034, in apply_metadata
File "site-packages\calibre\ebooks\metadata\opf3.py", line 385, in set_title
File "src/lxml/lxml.etree.pyx", line 1523, in lxml.etree._Element.makeelement (src\lxml\lxml.etree.c:58826)
File "src/lxml/apihelpers.pxi", line 105, in lxml.etree._makeElement (src\lxml\lxml.etree.c:17761)
File "src/lxml/apihelpers.pxi", line 1630, in lxml.etree._tagValidOrRaise (src\lxml\lxml.etree.c:34314)
ValueError: Invalid tag name u'dc:title'
Apparently, a guy managed to make an ePub without <title> (and quite a few errors in the OPF)
Calibre does not seem to be able to manage this absence (which was totally unlikely).