If I quit Neovim's containing terminal without exiting Vim first, then the next time I run Neovim I get the error:
E575: Error while reading ShaDa file: mark entry at position 2957 has
Press ENTER or type command to continue
I want to not see this error on startup.
To reproduce this error run nvim --cmd term
exit the terminal while still in vim and then run nvim --cmd term
again. This error is caused by not exiting vim before closing the terminal.
:h E575
and the error message you have corrupted data in your shada file. I'm not familiar with neovim but:h shada-file-name
seems to indicate where is your shada file. So find your shada file, find the corrupted data and no more "unavoidable error"