For what it's worth, I don't think it's that there aren't any examples where Cooper was at fault this season, I think it's that people rightly can't be bothered to trawl back through 20 highlight videos making a note of which goals we should cite on a Pasoti thread as being the fault of our rookie keeper.
Personally, I've seen countless examples throughout the season where all I've seen is a keeper rooted to the spot, usually on his goal line, after the ball has hit the net. Often it's after he's failed to collect a cross and he's just stood there throwing his arms up in the air like a child who's been told to turn the Playstation off. And no, I'm not going to provide specific examples but that's more of a sign that I simply can't be arsed to find them all!
To those who say he's destined for the top, I honestly can't see it and maybe that negates much of what I've said above - I'm not claiming to know everything about goalkeeping but, like Cobi said earlier in the thread, Cooper just doesn't seem to make the showcase saves - I don't remember seeing many saves where I've thought "Blimey, how did he get to that?" - quite often, for me, it's the complete opposite. He doesn't make stinking mistakes often; he just doesn't make the saves that some may think he could have done better with.
To conclude, I agree with the OP - not signing Palmer has cost us dearly this season, and I've been saying the same things for a good few months now. Here's hoping RL has a look at the loan market in January - if Luke's not the answer as a backup, when we clearly need a change of personnel in goal, then why on earth did we sign him? If it's purely as a mentor, well it's not working.