Except for those lucky few who use SEO to make huge amounts of money for themselves on a daily basis, most of us have to deal with clients - either internal & external. And a large part of dealing with those clients is about managing their expectations. Often people who don't really understand what we are or what we do will demand hard facts to determine the success of search marketing work. And these are often the ones that they will ask for, so I thought I'd point out how dangerous they can be...

1. Google Toolbar PageRank: I'm sure that we've all had a conversation along the lines of - "We've got a Google PageRank of 6 - that means we're great, right?", or "Our PageRank dropped - we're in the sh*t, right?" I find that there are a couple of ways to dampen down this sort of talk - when you say you've got a good PageRank, do you mean your homepage? What about your internal pages? And if your PageRank is dropping, is your traffic dropping too? NO? So what's the problem? Make sure clients understand that the PageRank they see is only an indicator, and probably out of date.
2. Alexa Rank: "I heard about this thing called Alexa, and since we've all downloaded it to check our rank, I've seen us shoot up the rankings - brilliant, eh?" Most people probably have no idea how Alexa works, or understand that downloading it and then visiting your site to check its ranking 10 times a day is like releasing a record and buying a hundred couples for your friends. Save Alexa for the trending people!
3. Google Rank: "Huh? What are you saying? Surely we all rely on using our rank as an indicator?" Well, maybe not any more. As others on this site have already discussed, the advent of truly personalized search means that ranking #1 when you've got a Gmail account and you're logged in might not mean so much any more. Only last week I had an (internal) client convinced that they were on the 1st page of Google for a search term when I couldn't see them anywhere - one quick log-out of Gmail, and we both had the same results (not on the 1st page, unfortunately!)
4. In-Bound Links: "OK - now you're just talking crazy!" Well, yes & no. We all know that Google uses links like I use Guinness (to keep it going from day to day), but once again, there's some perspective needed here. We all understand that links need to go to every page on a site that you want to rank; others may not. Next time someone tells you they've got thousands of links pointing at their site so why don't they rank, ask them if any of the links point at anything other than their homepage (and as for where the links come from, well, that's a whole different kettle of fish..)
5. Pages Indexed: "I've got 60,000 pages indexed in Google/Yahoo/MSN/delete as applicable..I'll be a millionaire by Xmas!" Yeah, or maybe no. So you have 60,000 pages in the index; how many pages does your site have? 60? Then you've got some serious duplicate content issues. 6,000,000? Then why is so little being indexed? People often assume that all you need in this game is numbers - it's our job to put those numbers in contact with reality.
6. Google Fight: I'm sure that everyone has seen the Google Fight game which checks two phrases to see which is mentioned more on pages indexed by Google, and then declares the one with the most to be the winner of the fight (sorry, Rand). Surely no one would use that as a serious measure of web success... You'd think not, but I saw a senior member of management, supposedly someone who 'got' the web, doing exactly that. He explained that if you have a brand online you should do this with your own and a competitor's brand, and if the competitor wins, you're in trouble. The fact that one site might have the brand name in the T&Cs at the bottom of the page, and another might not, seemed to pass him by completely - and this guy is meant to be web savvy. 

I hope that you can all relate to some of the points raised above and see the common message. Explaining what we do to people without resorting to technical jargon is incredibly important in order to manage expectations. Because, as all the examples above show (but particularly #6), a little knowledge can be a dangerous thing!