If you'd over-carbonated your beers would have gushed and you would have been unable to pour them easily.
If you'd got an infection, you'd have known it. It would have had any number of off-flavors and they would be getting worse. Google "infection flavors in beer" for possibilities, the biggies are diacetyl (rancid butter), butyric acid (vomit) and acetic acid (vinegar).
If you'd got a bit of oxygen in the beer when racking or at any other time on the cold side, it would have taken a while to show itself either as a wet paper like flavor or after even longer, reminiscent of cooking sherry.
And that was three things rather than two - I'll have to charge you half again the normal price... ;-)
One thing I advise frequently is to know what the effect will be of what you're doing. You mention full boil: It will yield a slightly lighter colored, slightly less melanoidin-flavored, slightly less hoppy beer if you don't adjust for the boil density, that is. None of those effects will ruin a beer. Might make a Hefeweizen too dark for competition or a reddish brown Pilsner but it won't ruin the beer. To be honest, I'd advise you to curl up with a good book, starting with "How to Brew" by John Palmer. If you know how to cause a thing, you have an idea as to how to prevent it.
Oh, one more thing before I end this rather lengthy post: You mention process of elimination. That's good, but to really do it you have to change only one thing at a time. My advice, get to a "good" beer, one that's drinkable, with as few variables as possible, meaning a simple Blonde, Pale Ale or something similar, then start your process of elimination by changing one factor at a time. You'll learn faster that way than by trying to cook a double imperial grapefruit IPA with twenty different hops. By brewing the latter, you'll never taste the effects of the change. Which makes me wonder if the reason they became so popular is that you don't have to be an outstanding brewer to make them.