Why did my cod turn into a hockey puck I have a way of cooking cod loin which has never failed me before.  This time, large parts of each loin became so rubbery as to be unchewable.  I know several things that deviated from my usual recipe, but I'm wondering which thing was the culprit.
First, the recipe:  Cod is dipped in milk and egg and then rolled in panko mixed with Cajun spices.  A little olive oil is heated in a cast iron skillet.  Cod is fried for a couple minutes on each side.  Then the heat is turned to low and the lid is put on the skillet.  The fish &quot;bakes&quot; in the cast iron for about 11 minutes.  This has never failed to give me nice white flaky cod.
A couple things were different this time:

These were from a bag of frozen cod loins (maybe from Sam's Club or HEB.)  These were the last two, so they'd been in the freezer a while.
I had gotten these last two out a couple weeks ago, and then didn't make the fish, so they went back into the freezer.
Instead of 11 minutes, they got more like 20 minutes of &quot;baking&quot;.

On the first piece of fish, I took a bite off the end and it was fine.  The next bite I couldn't cut with my fork.  It was almost exactly the texture of that triangular piece of cartilage one gets on a chicken breast: rubbery, tough, bleh.
I would think that if over-baking were the problem, then the fish would have been tough at the ends, but the ends were the only edible bits.    If it were that Sam's Club just stocks bad fish, then I'm incredibly unlucky/lucky in that out of a bag of 15 pieces, I chose two at a time over the months and only the last two were bad.  That leaves &quot;refreezing.&quot;  Only these two pieces were thawed and then refrozen.   Could that have caused the puck-ness?
A partial answer, to narrow down the possibilities.
I am pretty sure it's not overbaked. I have had overbaked fish, and while it does get dry, it doesn't get tough and rubbery. Rather, it starts falling apart, and the overall texture is quite chalky.
The &quot;puckness&quot; you describe does sound like it lost moisture in some ways, &quot;rubbery as to be unchewable&quot; and the comparison to cartilage are quite like the leathery quality of dried out fish.
The first culprit I am thinking of is freezer burn. If you didn't wrap the pieces perfectly during rethawing, and maybe even if you did, you may have gotten them freezer burned, or in this case I guess you could call it freeze-dried (although intentional freeze-drying would have used different conditions and would result in a different texture). This can readily account for the observed effect. The one detail that doesn't fit the hypothesis is your statement that the ends were better off than the middle - normally, the thinner a piece of fish, the more likely that it will dry out into that leathery state.
The second would be the rethawing itself. Normally it shouldn't have such drastic effects, but maybe you just had some kind of bad combination of conditions, or simply bad luck. I still find it less likely than the freezer burn explanation - while rethawing does cause a kind of dryness, it also contributes to mushiness, which is the opposite of the glued-together state you describe. But maybe it was the combination of freezer burn and rethawing such that each made the other worse.