PSA on Charge TMs and Secondary Charge Moves
Now that the PvP update is live, we have the option to unlock a second charge attack for each of our Mons (at a considerable cost). When you unlock the second charge move, if it's not the one you wanted, you can use a charge TM to change it. Since that Mon now has two moves, using the charge TM will bring up a prompt asking you which of the two moves you want to change.
This is the point where I advise that you click with care. As you can see in the screenshot, the buttons for the two moves you can change after using a charge TM are fairly close together. On top of that, the original charge move always seems to be highlighted, which draws more attention to it. Once you click a move, there is no secondary "are you sure?" prompt. The move just changes.
A simple misclick could easily result in a player accidentally TMing away a legacy move that they can't get back. I've already heard of a few horror stories where someone wanted to TM their Metagross so that it would have Earthquake as it's secondary charge move, and they accidentally TM away Meteor Mash instead.
I know how much legacy moves mean to all of us, so please be cautious when using those charge TMs, trainers!

Answers
Niantic should have a warning screen when you try to TM a legacy move, just like when you try to transfer an event pokemon, legendary pokemon, or mythic pokemon to the professor. That would prevent a lot of hair pulling after fat fingering the wrong button.
The worst I've done so far is to accidentally power up a level 35 Nidoqueen with awful IVs (wild caught). 8K dust I'd like back, but not something I can't overcome. Losing MM would be awful.