Anyone making up or selling kits should make an attempt to have the fabric pieces be as square as possible when cutting. Then, they should provide enough extra fabric so you can complete the kit, along with correct instructions. They should also proofread and "walk through" the instructions before packing the kits.
If something is wrong with the kit or with the instructions, they should stand behind the product 100%. It's not always possible in a BM shop, but in an online store, if a problem is found with a kit or instructions, the shop should proactively contact the remaining purchasers of that kit, explain and offer to fix the problem.
If your seller does not operate this way, it's time to shop elsewhere for your kits. If you have found a problem with the kit or instructions, you should contact the seller immediately. Reputable sellers will stand behind what they sell.