Such groups can currently be created by proc macros.

Reproduction, proc macro crate:

#[proc_macro]
pub fn add_mul(_: TokenStream) -> TokenStream {
    // ⟪ 2 + 2 ⟫ * 2
    vec![
        TokenTree::from(Group::new(Delimiter::None, "2 + 2".parse().unwrap())),
        TokenTree::from(Punct::new('*', Spacing::Alone)),
        TokenTree::from(Literal::u8_unsuffixed(2)),
    ].into_iter().collect()
}

User crate:

fn main() {
    let x = add_mul!();
    assert_eq!(x, 8); // FAIL: the result is 6 != 8
}

This is not a huge issue right now because proc macros have no reasons to created Delimiter::None delimiters themselves (but they can still get them from input and return them if they return their input partially or fully, see the example below).

However, this will became a huge issue when interpolated tokens like $e: expr migrate from AST pieces to token streams, because in the token stream model they are represented exactly like groups with Delimiter::None delimiters (proc macros already see them in this way).