When it's 24-9/16. Let me explain. I am building a custom guitar and wanted to use Gibson scale. I had everything done except the finish, but wasn't satisfied with the fretboard. I had free-handed the fret slots, and they were off here and there, so I decided to order a slotted fretboard from StewMac. Unbenownst to me, Gibson scale is mostly 24-9/16 for modern models, while 24-3/4 is mostly for certain vintage guitarsk. I never heard that before, and being a know it all I failed to read the copy at the bottom of the web page explaining all this. And since my bridge was already placed, there was no way I could make up for the 3/16" difference. What to do? Turns out, an earlier error came back to save me. I had underestimated how much length a 1/8" headstock overlay adds when it's canted 10 degrees. There is actually enough room for a 25" scale with the nut. So that fixes two errors at once and goes right along with the PRS inspired headstock. Lucky. I have made bodies before, but necks are a lot more complicated. It's good to learn from my mistakes, and I really think I lucked out on this. How about you? What guitar building fixes have you done, and have you lucked out like this before?