If there are inconsistencies or other problems with the instructions, I check with the scheduler before doing the shop. This gets me the info I need and lets them know there is an issue with the instructions. However, I think I once ran into an assignment that had so many issues between two different instruction sheets and the form that it would have taken more time and effort than the shop was worth to get it clarified, and I just cancelled it. Fortunately not many are that bad.
As far as spelling, grammar and punctuation mistakes, I would like to offer another option than pointing them out to an msc that might not take it kindly:
editmob.com
This site accepts corrections to on-line text and submits them under its own name to the offending site. They even have a drawing each month for their contributors, though I think it's just for a small gift card. I find this is a great way to vent frustration with msc double standards without putting my shopper reputation at risk.