RFC6020 ABNF grammar says:

revision-stmts = *(revision-stmt stmtsep)

stmtsep = *(WSP / line-break / unknown-statement)

Our YANG.g4 grammar says:

revision_stmts : (revision_stmt )* (stmtsep)*;

stmtsep : IDENTIFIER string? (stmtend | (LEFT_BRACE unknown_statement* RIGHT_BRACE));

So, in our grammar unknown-statement can occur only after all revision statements, but in RFC grammar unknown-statement can occur also before or between two revision statements etc.

For an example, parsing of following model fails:
module my-module {
namespace "my";
prefix my;

...

revision "2007-06-09"

my-prefix:my-extension 1;

revision "2008-06-09"

}