We've got a setup very similar to the picture in the answer to this:
I've read some similar questions here, but none is just like this:
The washer, whether it be flat or domed, on the valve next to the seat wears out by being crushed all the time, by requiring more pressure each time to close the hot valve completely to keep it from dripping, trickling, or leaking eventually becoming completely destroyed so as to require replacement. The water continues to come from the tub faucet head, as the diverter is left in the neutral position when not in use. They will come from the shower head eventually if left in that position, or the third option...
Replacing the seat once caused this not to happen for the longest period, but I just checked it, and it's still looking great/fine.
Any ideas? Using a 3/8" L type right now. Maybe there is some other solution that isn't quite obvious, or maybe a special material that won't just become completely more crushed with each turn. This last one lasted like a month...
Here's an image of the crushed dome washer
Here's the new flat style (rotating out the dome), installed: