Not so much a bug though unintended behavior due to game mechanics.
I rolled a Goblin and got the quest
Short Five Minute Adventure* in Sparkwater Port wich tasked me on getting certain items. Among others 5x
Copper Tube from a vendor in Razor Hill.
But they seem to be sold out all of the time. I've tried many times before abandoning this quest.
With that in mind I want to adress the following.
As this time my toon is level 60 and I am leveling my professions.
Now I'd love to make a Thorium Widget for
A Broken Trap and for leveling Engineering past 260 in general but that requires
Schematic: Thorium Widget wich is sold by only one vendor per side.
And this Schematic has a max stock of #1, on a very long timer.
This was fixed by Blizz in TBC by moving the schematic to the trainer, so you could train this at
Buzzek Bracketswing in Gadgetzan.
Now I can't speak for other professions but I can only speculate other professions have a certain item sold by a vendor wich holds progresson back, due to having just one stock on a long timer.
*This quest is listed in the database under 'The Barrens', this should be Durotar.