I already reported this bug to 1.12.2,but this bug is still relevant!

I was playing around with the LastOutput NBT Tag in CommandBlocks.
When you run this code

data merge block ~ ~1 ~ {LastOutput:"{{\"text\":\"This is a output message.\"}"}

with another (empty) CommandBlock on top, you will get an Java error message in the LastOutput panel.
The LastOutput Panel in the CommandBlock you just ran this command from doesn't show any errors. But ofcourse there is just one { to much!