My 'standard' mistake in the darkroom is to leave the enlarger in focus mode with the VC filters shut out, and then watch absolute mud come up in the developer. With the price of paper escalating as it is I can't afford to make any of those mistakes any longer!
But....this week I have finally, after storing a Logan 750 SimplexPlus Mat Cutter for about 5 years, set it up on the excellent print mounting table my wife built and taught myself to cut mats. The initial errors are comical, but I've gotten the hang of it nonetheless. "Aller anfang ist schwer" (I think?).