I got a good offer for search engine optimisation, which I'm thinking of taking up - what's the worst that can happen?
Search Engine Optimisation isn't an exact science, and requires a certain amount of experimentation. You need to be sure, though, that you, or whoever you're paying to optimise your site, plays by the rules. Otherwise, rather than simply performing badly in the search engine listings, you may find your site excluded altogether from mainstream search engines.
Let's explain this a little bit more. There are plenty of techniques to improve your search ratings for different keywords. Many of these techniques are simple common sense, good coding practice, and hard work. These techniques are often dubbed 'white hat' search engine optimisation.
There are, at any given time, a number of techniques that are of a questionable practice - which are termed 'black hat' s.e.o - and which may work extremely well for your site, for a time, until the search engines pick up on the trick. Black hat techniques may see you pick up plenty of visitors in the short term, but the likelihood is that your site will get banned (without notice) in the long term.
Ignorance is no excuse
The unfortunate problem here,then is that search engines don't take into account that you may have paid someone in good faith to improve your site's listings, without realising that their techniques are questionable.
An example of search engine optimisation going badly wrong occured when Google's anti-spam team removed pages from the BMW.de website. The german car manafacturer was penalised for presenting a keyword stuffed page to the google site crawler, while at the same time through some clever javascript usage sending a different looking page to actual visitors to the site (technically speaking, a doorway page).
So be aware what any s.e.o. work on your site entails. One guiding principle is that changes that are made should not include hiding text, or creating deceptive pages that look one way to a search engine, and another to your visitors.
If in doubt, ask your s.e.o. straight up to explain how their work relates to the published webmaster guidelines from search engines like Google.