Ah this reminds me of my first big mistake with PCBs. I have recently started down the hardware track, and my first PCB has a number of BC547 and BC558 transistors on it.

Once I had a functioning prototype, the next step was to convert it into a schematic. After that, you have to convert the schematic into a PCB. Now we are at two layers of translation, and at this step I made a mistake: I wanted to use SMD components to save money, and I found that the BC8xx transistors are the SMD equivalent of the BC5xx ones, so I used the footprint of the BC8xx transistors in my PCB, with no errors from KiCad.

As it turns out, the BC8xx footprint is not compatible with a BC5xx schematic! The pinout is different: the base is pin 1 instead of pin 2, so the components in my PCB that use transistors (crucially, the voltage controlled amplifier) didn't work. Unlike a bug, that mistake cost me $200 and weeks of development time, but after 10+ years of writing software I'm still happy to be making things that people can touch.