It is important as a pattern tester to take your duties seriously. Often designers trust in ultimate secrecy as there is a lot of hard work that has been put into a pattern and they would hate to see it wasted by a slip up on your part. After all, the launch is a huge part of a pattern/book.
My advice to those testers out there is this: Be honest. Knit the pattern as is; don't change anything (not even one little stitch!)even if you know how to fix the problem easily. It is up to the designer to choose how to apply corrections. Be tasteful in your critique and if you have 10 pages worth of notes, let them know everything. After all, they are asking for your expertise and are relying on you. Make a point from the beginning to define if the designer would prefer regular updates or one update when you are finished. Be sure you both agree on a due date as well as any payment or yarn supplies before you cast on.
In a pattern testing kit, it is helpful to keep everything together in one place so you are not scrambling for your notes should the designer ring up and say "You know on row 22....I'm thinking of changing that!". Keep the pattern, a notebook, coloured pens or highlighter (to make changes more obvious) and any tools you will require for the knitting in a simple case. This makes it easy to grab on your way out the door, or for those of you like me with a 1 hour commute, easier to manage on the bus.