This seems to be due to
https://github.com/emacs-mirror/emacs/commit/dfc850ca0223ffd1d2048657ea6d20c971865d51

specifically (backward-up-list 1 t t) seems to emit that error message:

                ;; Assume called interactively; don't signal an error.
                (user-error "At top level")


Easy reproduction: edit a csharp file, go to some toplevel line (e.g.
"using System;" that is pervasive in a lot of C# code) and run
c-indent-line-or-region.


By the way: (advice-add 'c-guess-basic-syntax :around
#'csharp-guess-basic-syntax)
means that csharp-guess-basic-syntax runs also in all other c-mode derived
modes.  So this error also happens if you first edit a .cs file and then a
normal .c file and you try to, for example, indent a toplevel comment in
the C code.  Very frustrating.

-Aleksey