diff --git a/src/pkg/go/printer/printer.go b/src/pkg/go/printer/printer.go
index 030bc2387e..fa591c5764 100644
--- a/src/pkg/go/printer/printer.go
+++ b/src/pkg/go/printer/printer.go
@@ -225,7 +225,14 @@ func (p *printer) writeString(pos token.Position, s string, isLit bool) {
 		// (used when printing merged ASTs of different files
 		// e.g., the result of ast.MergePackageFiles)
 		if p.last.IsValid() && p.last.Filename != pos.Filename {
-			p.indent = 0
+			// Note: Do not set p.indent to 0 - this seems to be a bad heuristic.
+			//       ASTs may be created by various tools and built from nodes of
+			//       different files. An incorrectly constructed AST will likely
+			//       not print at all, but a (structurally) correct AST with bad
+			//       position information should still print structurally correct.
+			//       If p.indent is reset, indentation may be off, and likely lead
+			//       to indentation underflow (to detect set: debug = true).
+			//       See also issue 4300 (11/16/2012).
 			p.mode = 0
 			p.wsbuf = p.wsbuf[0:0]
 		}