I would say that the ripper is my best friend, but how could I hate my best friend?!
A while back I started a quilt where I sewed each block and then trimmed the edges to the proper dimension. STUPID ME, the blocks could not be sewn together properly because I cut off my 1/4" on some and had more like a 1/3" on others! I couldn't even use a ripper to help me out on that one! I was so discouraged that it is still sitting in the box waiting to be finished.