The first obvious mistake you should be aware of is using a template
that is very popular. If many people use the same template, your website will
not appear unique at all and your credibility as a solid, different website
will be tarnished. In other words, you will appear generic just like your
next-door neighbours.
To whole point of using a web template is to save time and effort. You
just change the title and appropriate details and you're done.
The biggest
mistake one makes is to customize the template beyond recognisation. While that
may be good in the sense that you're creating a unique graphic, you're defying
the very purpose of using a web template -- saving time and effort.
However, on the opposite side, if a template you purchase is suitable
but some changes must be made to suit your site's theme, then you will have to
take some time to make the changes.
For example, you can find a very nice
template that suits your hobby site except the original designer has put an
image of stamps in the header. You can find images of garden plants and spades
to replace the stamps for your gardening hobby site. However, do only make the
necessary changes and don't redesign the whole template.
In some circumstances, some people simply make the wrong choice of
templates. This is a very subjective issue but you have to be careful in
selecting templates to suit your audience. Do not choose templates just because
they are pretty, choose them because they serve your purpose.