Here's a classic: toss all negative results.
Clearly, everything we do is positive, right?
Nope. Anything that can have an effect can have a negative effect.
(I've met a number of marketing people that really truly believe that people wait at home looking forward to their telemarketing calls. And that calling something 'viral' in a powerpoint is enough to actually create a viral marketing campaign).
There's another factor. Depressingly, a lot of marketing campaigns do absolutely nothing. Random noise takes over; half will be a little positive and half will a little negative. Toss the negative results and you're left with a bunch of positive results. Add them up and suddenly you've got significant positive results from random noise. This is bad.
I've seen an interesting variant on this technique from a very well-paid consultant. Said VWPC analyzed 20 different campaigns and reported extensively on the one campaign that had results that were significant at a 5% level.