In general: Not knowing what the {{Reflist}} template is, what it does, and that it is necessary if there are anycitations present.
Adding citations and forgetting to add the {{Reflist}} template.
Placing <ref> tags below the {{Reflist}} template, which always causes an error.
Forgetting to place the </ref> tag at the end of the citation... There cannot be <ref> without </ref>. Remember that.
Not knowing that <references/> is the same as {{Reflist}}.
Adding something other than a URL to the url= parameter.
Not using or ignoring the {{Reflist}} template, and manually numbering the references in the references section.
Adding a citation but not including the title= parameter in the citation. Nice.
Thinking that every single citation needs a reference group. For example, <ref name="This website" group="This website">This website</ref> Wrong. Unless citing a 'note', group="This website".
Adding a reference group such as 'note', and forgetting to place the {{Reflist|group=note}} in the appropriate ==Notes== section in the article.
Using the Cite web template and not adding a URL. You're citing a website, why not tell us which one?
Adding the accessdate= parameter in a citation, but not including a URL. Accessdates are meaningless without the URL they belong to.
Using the archiveurl= parameter, but not including the original url= parameter. How did you find the archive if you didn't know the URL?
Forgetting the archivedate parameter when using an archiveurl. It's understandable.
Forgetting to add http:// to the beginning of the URL.
Writing a citation like this: <ref>{{cite web|url=So-and-so did this|title=http://this-address.com}}</ref>. You swapped the parameter names.
Thinking that the laysummary= parameter means an actual summary of the source. It doesn't. You need a layurl=.
