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Entering http://en.wikipedia.org/wiki/Special:ExpandTemplates, setting PAGENAME to Template:Foo, and content to {{Db-t1}}, it will fail on expanding the ombox template, resulting in:
{{
  ombox   <!--"demospace=other" or a new namespace--><!--End switch-->
| type       = speedy
...

As the template in question has been changed, I have made an more static example, at [[en:User:AzaToth/Expand]], use {{User:AzaToth/Expand}} and test both with and without remove comments

Expected, not a bug. If you want it to work, don't click it.