For example, on heavily pressured waters where a lot of people fish gold bead pheasant tails, will that pattern slowly become less effective throughout the season as the trout get more exposed to it.

I know that trout have good short term memory, if I fail a strike on a Cicada fly, they will unlikely take that fly again, but if I change to say a Chernobyl ant or stimulator chances are they will have a second go. So atleast in the short term,trout can recognize and avoid specific patterns. But how long does that memory last. 

On another occasion, late season, there was a big jack guarding a bunch of hens so I tied on a globug and it took it. I lost it as I was netting it. The jack went back to his harem and I cast again with same fly within 5 minutes of trying to net it and it took the fly - this time netted properly and released it. Almost unbelievably, I caught it a 3rd time - 3 times in about 20 minutes!! 

Browns do eat egg patterns as saw some getting caught on the Waitahanui 2 days ago.