A few years ago, I had a chance at a great deal on an aluminum frame Commencal mountain bike that had a big dent on the downtube when we received it at our warehouse. It was a sample, so it was never really intended for sale, more so that we could show it to dealers. Eventually, we lost the distributorship and had to sell off remaining stock, so I bought it. It turned out that the dent wasn't the problem, the frame had at least 2 major flaws in both the front and rear triangles, neither of which really had much to do with welding but more to do with poor quality control and testing. A dent may not kill your frame, but poor quality control and insufficient testing can be a killer