Recently dumped a batch of Oktoberfest that I did a triple decoction on. Fast forward 3.5 months after 3 months of lagering, getting ready to keg and the sample still tasted like a yeast bomb. Tasted like that during transfer to secondary but figured it would go away after lagering NOPE. Don't know what happened, tried new yeast ( both batches I've made with Mangrove Jack have been dumpers) or was it the decoction ( done it several times with great success), did I screw up with ferm temps ( don't think so as I brew way more lagers than ales), under/over pitch ( maybe used two rehydrated packs for a 5.5gal batch, I always use 2 rehydrated packs for 5.5 gal lager).....long story short I was severely disappointed as a Oktoberfest is on of my top 5 favorite beer styles.
Another dumper was a Scottish Ale where I used Mangrove Jack yeast.
Had one or two others early on in my brewing that were just awful beers mostly due to water issues. If it's good enough water to drink you can brew with it, not in my case, RO water since then.