Good point g-man.
In fact, some bacteria, like acetic acid bacteria, can form aldehydes by oxidizing some of the alcohol produced by the yeast. Some yeasts produce those as well. That's part of what gives a fino its distinctive nose so if you don't get the kind of caramel notes you'd get from open-air oxidiation, it's also a kind of oxidation but the result is based on the yeast or bacteria as g-man said.
And just as he points out, that's not something you take care of by using a better stopper - dirty facilities at any point along the line can do it. If it's in the winery, it can be expensive and tough to clean out.