I can say with a lot of assurance that the most common error made by people who are following tables and no computer is ascending too slowly to the first decompression stop, and I am not talking about being off by 10-15 seconds. In the case noted above, the divers should have reached their first stop in about 30-45 seconds. It took them over 3 minutes, at which time they started counting their stops according to their plan, not realizing that they had in essence added 3 minutes of bottom time to the dive. That was one of their mistakes.
It happens to people with computers as well, but in that case the computer adjusts for it. I was on a dive on which my buddy and I had the same algorithm and dive plan as another pair of divers sitting next to us on the boar, as was communicated to the DM ahead of the dive. As expected, we arrived at the ascent line at about the same time. My buddy and I ascended first, and when we got to our first stop, I looked down and did not see the other pair of divers. I was worried. The first stops were not long, and we did not see them below us until our third stop. We got back on the boat, the next to last group to do so. As we were putting our gear away, the DM came to us to ask about the last group. We assured him they were on the line, but we had no idea what was holding them up. When they finally got on the boat, they told the DM they did not know what had happened. They had arrived at the ascent line at the planned time, but as they ascended, their computers just kept adding deco time. I then remarked that they had done their early ascent very slowly, and one of them looked at me like I was crazy. "You're supposed to ascend slowly!" he said.