X-posted to legos

My son was working on the new Harry Potter train, and after a few steps found that he had used 1x3s instead of 1x4s on the first layer. We had to shift everything one unit down and replace the 1x3s (which belonged on the second layer.) All I could think of was, he's experienced his first "off-by-one" bug. But I knew nobody else in the house would appreciate the joke.