While not a problem with american classic and (mostly) vintage cars, our British car bretheren with older cars face a huge challenge with the "standard" fasteners used on their cars.
Back in 1841, Joseph Whitworth established the worlds first "standard" for nuts and bolts. Not long after that, the British government adopted it as a standard named the British Standard Whitworth (BSW). These nuts and bolts were the earliest mass produced nuts and bolts that were interchangeable, regardless of where they were manufactured. While this seems seems like a no-brainer, back in the day that was far from the case. Nuts and bolts were made with a lathe back then, and cutting threads was not a simple affair. Everybody that made bolts made them slightly differently and nothing interchanged with somebody elses product - until the advent of the whitworth.
Mostly found on old British cars, whitworth sizes were based on the shank diameter of the bolt (vs. the head size used today). Also, the threads were slightly different, most notably having a radius in the extremeties of the trough and ridge areas instead of a flat area found on todays fasteners. At any rate, nothing modern works with whitworth nuts and bolts and todays wrenches don't fith them either!
Since nothing fits them quite right, it takes a unique set of whitworth wrenches to do much of any mechanic work on cars made with these fasteners (think old MGs, Morgans, ACs, etc.). The wrenches are crazy expensive and in limited supply.
So the next time you happen to find an old nut or bolt that doesn't seem to fit any wrench in your box, you have probably stumbled upon a piece of automotive history. Knowledge of the BSW is worth a whit (or at least explains why nothing works!).