Preferred vendors can be a problem. The purchasing people have their favorites, and the engineer may not always get the desired part. I had requested some optical patch cords from a certain vendor; purchasing went with a different vendor who had supplied copper cables in the past. One of the fiber cables still had the unpolished end fiber sticking out of the connector -- so much for quality control.
A similar quality problem occurred when I specified a certain brand of twisted-pair cable for a production test fixture. My own test fixture worked nicely with the brand I chose, but production was swayed by purchasing to go with its preferred brand. There was a huge variation in attenuation between pairs in the same cable. Let the losses begin.
Sometimes, you can't get parts with multiple sources, or even a second source. Trying to explain to the components group that a certain brand of optical transmitter and receiver was the only one available at a reasonable price did not stop them from whining -- until they did some of their own research and determined that there really was no choice.
I did see their point, especially when that same vendor made a process change that resulted in a horrendous optical overshoot in the transmitter. I had to come up with a fix that required a large value trimmer capacitor, and because the value I needed was a year lead-time (after I bought about 400 of them myself), purchasing wouldn't allow it. We ended up using a varactor diode and a trimpot instead.
The most difficult component hassle was with a simple 32.768kHz crystal and the associated CD4060B CMOS oscillator/timer IC. No way were the component cops going to allow something as obsolete as the CMOS 4000 series in a new design! It took a lot of explaining that, old or not, it was not about to go obsolete and that the chip had recently been made available in a surface mount package. I was certainly not about to design in a separate crystal oscillator and counter chain.
The crystal itself was another problem. What's so hard about procuring a 32.768kHz tuning-fork crystal that is made in the billions? None, except for the fact that the corporate-preferred vendor had none in stock and there would be a twelve month lead time. I sent a "nastygram" to the entire department explaining that I could not sit around for a year and that the component police needed to find and approve a crystal now. They did.
Have you ever been a component engineer? Let's hear your side of the story.