Everyone is saying water, which I would totally agree is a possibility. It's pretty easy to go with RO water bought from a store and generally doesn't cost much (stores like Walmart often have a kiosk where you can refill jugs for an even lower price - I get 8 gallons of RO water for around $3 and know of a place that's kinda out of the way where I could get that much for less than $2...cheap insurance that a batch won't turn out bad because of water).
However, another possibility that I haven't really seen mentioned, other than ferment temperatures, is yeast. I've found if I use S-04 (a popular yeast and one that should work very well in a porter), temp control is desperately important. Anything above low 60s and it's just plain nasty - and I've gotten a metallic flavor from a porter fermented in the high 60s. If you used S-04 and it got too warm, it could be that as well, not just the water. At one time I vowed I'd never touch S-04 again but then I gave it one more shot, keeping it around 62 for the first 3-4 days of fermentation for an IPA, and it turned out great. So be aware if you used that yeast, IMO it HAS to be kept cool, moreso than other dry yeasts.