Today on a campus fiber job, we encountered a configuration that was passing traffic but was not engineered according to fiber standards. The backbone fiber between buildings was single-mode, but the patch cords connecting the fiber panel to the switch were OM3 multimode jumpers. Single-mode and multimode fiber should not be mixed. However, in some cases, the link can still appear to work. Here’s why: if the switch is using single-mode optics (1310 nm), light can pass thro