Using "at" in a multi-line macro that is itself used as the data parameter of another "at" leaves the inner "at" unexpanded, leading to errors. In the following, the -DERROR -E run shows "..@11.strucstart:" \ "at bar.bar, db 0" which means the inner "at" wasn't expanded, and is then subsequently passed as a literal instruction to the assembler.

$ cat test2.asm

	struc foo
.foo:	resb 1
	endstruc

	struc bar
.bar:	resb 1
	endstruc

	%macro quux 0
	istruc bar
at bar.bar,	db 0
	iend
	%endmacro

	istruc foo
%ifdef ERROR
at foo.foo,	quux
%else
at foo.foo
		quux
%endif
	iend
$ nasm test2.asm
$ nasm test2.asm -DERROR
test2.asm:18: error: parser: instruction expected
$ nasm test2.asm -E
%line 2+1 test2.asm
[absolute 0]
%line 2+0 test2.asm
foo:
%line 3+1 test2.asm
.foo: resb 1
foo_size equ ($-foo)
%line 4+0 test2.asm
[section .text]
%line 5+1 test2.asm

[absolute 0]
%line 6+0 test2.asm
bar:
%line 7+1 test2.asm
.bar: resb 1
bar_size equ ($-bar)
%line 8+0 test2.asm
[section .text]
%line 9+1 test2.asm

%line 15+1 test2.asm

..@7.strucstart:
%line 20+1 test2.asm
times (foo.foo-foo)-($-..@7.strucstart) db 0
%line 20+0 test2.asm

%line 11+0 test2.asm
..@11.strucstart:
%line 12+1 test2.asm
times (bar.bar-bar)-($-..@11.strucstart) db 0
%line 12+0 test2.asm
db 0
%line 13+1 test2.asm
times bar_size-($-..@11.strucstart) db 0
%line 23+1 test2.asm
times foo_size-($-..@7.strucstart) db 0
$ nasm test2.asm -DERROR -E
%line 2+1 test2.asm
[absolute 0]
%line 2+0 test2.asm
foo:
%line 3+1 test2.asm
.foo: resb 1
foo_size equ ($-foo)
%line 4+0 test2.asm
[section .text]
%line 5+1 test2.asm

[absolute 0]
%line 6+0 test2.asm
bar:
%line 7+1 test2.asm
.bar: resb 1
bar_size equ ($-bar)
%line 8+0 test2.asm
[section .text]
%line 9+1 test2.asm

%line 15+1 test2.asm

..@7.strucstart:

times (foo.foo-foo)-($-..@7.strucstart) db 0
%line 18+0 test2.asm
..@11.strucstart:
at bar.bar, db 0
times bar_size-($-..@11.strucstart) db 0
%line 23+0 test2.asm
times foo_size-($-..@7.strucstart) db 0
$ nasm --version
NASM version 2.15.03rc1 compiled on Sep 29 2020
$