Three batches come to mind. Batch #4, which I created with a can of Hopped LME, bunch of DME, a bunch of left over hop pellets and some random dry yeast packets. I bottled the mess a few weeks later. The beer wasn't that horrid, it just geysered out when you opened the bottle.
Batch #24, Pilsner, Metallic-Band-aid taste. I theorized that it was from a corroded Wort chiller and excessive use of bleach as a cleanser.
Batch #36, Spiced Strong English Ale, Nothing wrong with the Mash, Sparge, Boil or chilling. The wort was wonderful, the yeast was not up to the challenge. It never did get going and eventually a bacteria took over and turned the whole batch sour. Nasty vinegar tasting stuff. YUK. I'll be brewing batch #36 again next weekend. (even though I'm almost ready to bottle batch #37 and #38.)