nopCommerce 3.0.0 beta has a topic 'PageNotFound' where a custom 404 message could be included
The default contains:
<li>If you typed the URL directly, please make sure the spelling is correct.</li>
but it should be
<li>If you typed the URL directly, please make sure it is correctly written</li>
because the term "spelling" refers to correct spelled words and not a correct written URL. Even a URL with misspelled words (maybe intentionally) must be written correctly to get the page.
Another issue:
All headlines, menu items .. are lower case. As a German i know the non native learn at school that all words are lowercase except of... OK that is fine, but not for headlines, menu items and so on. Several decades ago U.S. newspaper began to emphasize headlines by typing the headwords uppercase.
nopCommerce should consider to follow this trend as almost all magazines and many bloggers do these days.
A little research, but this guy prefers lowercase (downstyle in his own terms):
http://www.davemulder.com/articles/article-titles-upper-case-versus-lower-case/
B.T.W. Wikipedia and eBay uses the lowercase variant, but Amazon Youtube, Google and Facebook uses the uppercase variant for titles, categories etc. Sometimes they mix it up and use uppercase only for H1 and lowercase for H2 headlines.
It is a question of "house style".