Plan for a mistake or a buffer in DIY projects

Murphy's law may strike at any time. Obviously we never want to think this will happen at our own weddings.
But from one 3 month planning bride who wishes she had more time, let me tell you, there will be times where you'll have to replan things due to mistakes or other changes (and they won't be your fault but you'll have to correct them).

Examples in the last few weeks:

• Our programs got miscut by Kinko's and the text for our best man's name cut off (I was an upset bridezilla inside only) but I calmly showed them, they are reprinting and recutting, and I am picking them up today
• The last batch of cookies my bridesmaid and I made was left in too long and is too crispy looking now (we were busy trying to wrap the other cookies) but luckily I have an extra batch of dough. The first batch was in a minute too short and is more cakey.
  
• I totally miscut a piece of scrapbooking paper for a DIY coaster project and don't have any paper I can use from that sheet
  

It's not the end of the world if a project doesn't go right. Hopefully you have time to make more. Or like my first batch of cakey cookies, we now have cute shapes for each favor box.

Hopefully your projects go well! What have you had to replan due to something you or one of your vendors did?