While the broader market gained around 0.4% in the last year, IPG Photonics shareholders lost 56%. However, keep in mind that even the best stocks will sometimes underperform the market over a twelve month period. Unfortunately, last year's performance may indicate unresolved challenges, given that it was worse than the annualised loss of 3% over the last half decade. We realise that Baron Rothschild has said investors should "buy when there is blood on the streets", but we caution that investors should first be sure they are buying a high quality business. I find it very interesting to look at share price over the long term as a proxy for business performance. But to truly gain insight, we need to consider other information, too. For example, we've discovered 1 warning sign for IPG Photonics that you should be aware of before investing here.