Could be a number of things causing off flavors, but if you have a routine, are fairly consistent with what you use to brew with, how you keep clean and sanitary, and aren't violent with the finished product, things don't start going wrong out of nowhere.
Now if you're like me and haven't made a consistent product yet because there are so many different beers to be made, finding a specific oddity is difficult to find. Sometimes just the style of beer we make can hide a flavor that wasn't noticed until a different beer is made. Hoppy IPA's and bold dark Porters and Stouts can hide a lot of problems and mistakes.
Metallic flavors*in your beers are perceived as the taste of a rusty nail, or coin-like, tinny and blood-like.*One source of these off-flavors is from aluminum pots or other un-plated steel surfaces. High iron concentrations in the brewing water can account for some of these flavors as well.
When stainless steel is cleaned without passivating the surface (oxidizing to produce a layer of protective oxide on the surface) the unprotected steel can also cause off flavors.
Check the quality of your bottle caps, filter your water if necessary or use bottled water if you must, and keep all of your grain stored under proper conditions to prevent or reduce the coin-like off-flavors in your beer.
Hope this helps.