I don't have a personal blunder to share, but my brother recently started building his first plane, and has made a couple of neat mistakes...

His first mistake was when he tried to build the left and right sides of the cowling... He laid both pieces down on the table and glued the mounting ribs to them. When he was finished he had two exact replicas rather than mirror images, so one side had the rib inside as planned, but the other had the rib on the outside...

Most builders just monokote the wings but my bro tried to carpet his when he set it down on the floor to let the glue dry... Needless to say, he had to cut it free from the rug with an Xacto... He thought he had done a good job with minimal damage to the carpet, but as soon as his wife got home, she noticed something wrong with it... As soon as he opened his mouth he was dead... I was rolling on the floor laughing when he told me these...