This means you are using invalid markup on your signature. Some possible causes with their corresponding solutions:
;Unclosed tags :If you are opening a tag without the corresponding closing tag (for instance:<br /> <code><nowiki>[[User:Example|<span style="color:white">Example]]</nowiki>),<br/code>), you should close the tag (for instance: <code><nowiki>[[User:Example|<span style="color:white">Example</span>]]</nowiki>).<br/code>). It's also a good idea to put the tags outside the link if possible (for instance: <code><nowiki><span style="font-family: serif">[[User:Example|Example]]</span></nowiki></code>).
;Mismatched or incorrectly nested tags :If the tags are mismatched (for instance: <code><nowiki><s><u>hi</s></u></nowiki></code>), fix them (for instance: <code><nowiki><s><u>hi</u></s></nowiki></code>).
;Unquoted attributes :It's also recommended to use quotes on all attributes (for.<br/>For instance, use <code><nowiki><span style="color:white"></span></nowiki></code> instead of<br /> <code><nowiki><span style=color:white></span></nowiki></code>).
;Unclosed entities :If you have aan [[HTML entity]] which is lacking the final <codenowiki>;</codenowiki>, you need to add it; if you have a bare <codenowiki>&</codenowiki>, it must be replaced by <codenowiki>&amp;</codenowiki> (a bare <codenowiki>&</codenowiki> is always a mistake in either HTML or wikicode).
;Unescaped special characters :If you are using one of <codenowiki>&</codenowiki>, <codenowiki><</codenowiki>, or <codenowiki>></codenowiki>, and want it shown as text, it must be escaped as <codenowiki>&amp;</codenowiki>, <codenowiki>&lt;</codenowiki>, or <codenowiki>&gt;</codenowiki>, respectively.
* Do not use a table. A table causes the signature to display on more than one line. Use span instead.
==Wikilink behavior==
Remember that ''all'' wikilinks show up as [[{{FULLPAGENAME}}|bold black text]] on the page they link to,. soSo if you are testing your signature on your userpage/talk page and the user/talk part of it doesn't show up as a link, that may be why.
==A note on long signatures==
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