A line looks a little light, so you go back over it. It seems reasonable enough, after all, the line could use a little help, but the glass doesn’t respond in the same way paper or wood would. The first pass of your cutter has already created a microscopic surface crack in the glass. Passing the wheel over the same line again can result in crushing the glass, creating a second fracture next to the first, and/or making the intended break line more unpredictable.
All of this is problematic because you are not cutting all the way through the glass—you’re scoring the glass to allow for a controlled break. One clean pass creates one path along which the glass will easily follow once pressure is applied. Going over the line again, even with just a slight change in the angle of the cutter, can easily result in multiple break lines that are close together. Once you’ve placed your breaking pliers, you’re applying pressure over an area that has already been made more complex than necessary. The glass can now chip, break away from the pattern line, or break in such a way that it leaves behind a lot more grinding.
Where did this temptation come from? I think it often starts with being unsure about how hard to press when making a cut. A very heavy line looks stronger than a light one, and conversely, a light line can feel weak. But appearance can be deceiving, especially on certain types of glass. What really matters is not the look of the score, but whether the cutter moved smoothly all the way from one end of the desired cut to the other. Even if you can see the line clearly, stopping short, increasing pressure in one area, or constantly re-adjusting the cutter can all make for a broken fracture.
If you want to see the difference between one pass and two passes in a tangible way, use some scrap glass. Make one straight score using one smooth pass, then make another straight score in another piece of glass and run the cutter right over the line a second time. Break them both in the same way and observe the resulting edges. The piece with two scores might have some small crushed areas or an uneven fracture right alongside the initial score. Even if both break well, take a closer look at the surface. The idea here is not necessarily to avoid score lines that need to be corrected, but rather to understand why you should only pass the cutter over the glass once.
When the first score really is no good, don’t try to “fix” it by running the cutter over the existing score multiple times. Consider that scrap glass to be your lesson, and make a note about what went wrong. Did your hand drift off the ruler? Did the cutter slow down midway? Was there a change in pressure? Then, take another piece of glass and make a new score. Learning from the mistake is more useful than trying to make the same bad score work.
This lesson is especially important when scoring a curved line. Curves already require more control, so passing the wheel over the line a second time only adds more unpredictability. Keep to a single, deliberate score, break the glass, and grind only as much as you absolutely must. If the line needs adjustment, cut it off and make a new one rather than teaching yourself that every questionable score needs a second pass.