diff --git a/src/pkg/database/sql/fakedb_test.go b/src/pkg/database/sql/fakedb_test.go
index 3bbbb430b49..8732d028bc5 100644
--- a/src/pkg/database/sql/fakedb_test.go
+++ b/src/pkg/database/sql/fakedb_test.go
@@ -209,10 +209,10 @@ func (c *fakeConn) Begin() (driver.Tx, error) {
 
 func (c *fakeConn) Close() error {
 	if c.currTx != nil {
-		return errors.New("can't close; in a Transaction")
+		return errors.New("can't close fakeConn; in a Transaction")
 	}
 	if c.db == nil {
-		return errors.New("can't close; already closed")
+		return errors.New("can't close fakeConn; already closed")
 	}
 	c.db = nil
 	return nil
diff --git a/src/pkg/database/sql/sql.go b/src/pkg/database/sql/sql.go
index afee275c358..c00425d8fa1 100644
--- a/src/pkg/database/sql/sql.go
+++ b/src/pkg/database/sql/sql.go
@@ -262,6 +262,9 @@ func (db *DB) connIfFree(wanted driver.Conn) (conn driver.Conn, ok bool) {
 	return nil, false
 }
 
+// putConnHook is a hook for testing.
+var putConnHook func(*DB, driver.Conn)
+
 // putConn adds a connection to the db's free pool.
 // err is optionally the last error that occured on this connection.
 func (db *DB) putConn(c driver.Conn, err error) {
@@ -270,6 +273,9 @@ func (db *DB) putConn(c driver.Conn, err error) {
 		return
 	}
 	db.mu.Lock()
+	if putConnHook != nil {
+		putConnHook(db, c)
+	}
 	if n := len(db.freeConn); !db.closed && n < db.maxIdleConns() {
 		db.freeConn = append(db.freeConn, c)
 		db.mu.Unlock()
@@ -654,7 +660,7 @@ func (s *Stmt) Exec(args ...interface{}) (Result, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer releaseConn()
+	defer releaseConn(nil)
 
 	// -1 means the driver doesn't know how to count the number of
 	// placeholders, so we won't sanity check input here and instead let the
@@ -717,7 +723,7 @@ func (s *Stmt) Exec(args ...interface{}) (Result, error) {
 // connStmt returns a free driver connection on which to execute the
 // statement, a function to call to release the connection, and a
 // statement bound to that connection.
-func (s *Stmt) connStmt() (ci driver.Conn, releaseConn func(), si driver.Stmt, err error) {
+func (s *Stmt) connStmt() (ci driver.Conn, releaseConn func(error), si driver.Stmt, err error) {
 	if err = s.stickyErr; err != nil {
 		return
 	}
@@ -736,7 +742,7 @@ func (s *Stmt) connStmt() (ci driver.Conn, releaseConn func(), si driver.Stmt, e
 		if err != nil {
 			return
 		}
-		releaseConn = func() { s.tx.releaseConn() }
+		releaseConn = func(error) { s.tx.releaseConn() }
 		return ci, releaseConn, s.txsi, nil
 	}
 
@@ -776,7 +782,7 @@ func (s *Stmt) connStmt() (ci driver.Conn, releaseConn func(), si driver.Stmt, e
 	}
 
 	conn := cs.ci
-	releaseConn = func() { s.db.putConn(conn, nil) }
+	releaseConn = func(err error) { s.db.putConn(conn, err) }
 	return conn, releaseConn, cs.si, nil
 }
 
@@ -800,7 +806,7 @@ func (s *Stmt) Query(args ...interface{}) (*Rows, error) {
 	}
 	rowsi, err := si.Query(sargs)
 	if err != nil {
-		s.db.putConn(ci, err)
+		releaseConn(err)
 		return nil, err
 	}
 	// Note: ownership of ci passes to the *Rows, to be freed
@@ -878,7 +884,7 @@ func (s *Stmt) Close() error {
 type Rows struct {
 	db          *DB
 	ci          driver.Conn // owned; must call putconn when closed to release
-	releaseConn func()
+	releaseConn func(error)
 	rowsi       driver.Rows
 
 	closed    bool
@@ -990,7 +996,7 @@ func (rs *Rows) Close() error {
 	}
 	rs.closed = true
 	err := rs.rowsi.Close()
-	rs.releaseConn()
+	rs.releaseConn(err)
 	if rs.closeStmt != nil {
 		rs.closeStmt.Close()
 	}
diff --git a/src/pkg/database/sql/sql_test.go b/src/pkg/database/sql/sql_test.go
index 02ab20cd7c1..90a40efa286 100644
--- a/src/pkg/database/sql/sql_test.go
+++ b/src/pkg/database/sql/sql_test.go
@@ -5,13 +5,35 @@
 package sql
 
 import (
+	"database/sql/driver"
 	"fmt"
 	"reflect"
+	"runtime"
 	"strings"
 	"testing"
 	"time"
 )
 
+func init() {
+	type dbConn struct {
+		db *DB
+		c  driver.Conn
+	}
+	freedFrom := make(map[dbConn]string)
+	putConnHook = func(db *DB, c driver.Conn) {
+		for _, oc := range db.freeConn {
+			if oc == c {
+				// print before panic, as panic may get lost due to conflicting panic
+				// (all goroutines asleep) elsewhere, since we might not unlock
+				// the mutex in freeConn here.
+				println("double free of conn. conflicts are:\nA) " + freedFrom[dbConn{db, c}] + "\n\nand\nB) " + stack())
+				panic("double free of conn.")
+			}
+		}
+		freedFrom[dbConn{db, c}] = stack()
+	}
+}
+
 const fakeDBName = "foo"
 
 var chrisBirthday = time.Unix(123456789, 0)
@@ -358,6 +380,22 @@ func TestTxQuery(t *testing.T) {
 	}
 }
 
+func TestTxQueryInvalid(t *testing.T) {
+	db := newTestDB(t, "")
+	defer closeDB(t, db)
+
+	tx, err := db.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer tx.Rollback()
+
+	_, err = tx.Query("SELECT|t1|name|")
+	if err == nil {
+		t.Fatal("Error expected")
+	}
+}
+
 // Tests fix for issue 2542, that we release a lock when querying on
 // a closed connection.
 func TestIssue2542Deadlock(t *testing.T) {
@@ -562,3 +600,8 @@ func nullTestRun(t *testing.T, spec nullTestSpec) {
 		}
 	}
 }
+
+func stack() string {
+	buf := make([]byte, 1024)
+	return string(buf[:runtime.Stack(buf, false)])
+}