diff --git a/docs/rh-log.txt b/docs/rh-log.txt
index 8bddbe966..cad6765ee 100644
--- a/docs/rh-log.txt
+++ b/docs/rh-log.txt
@@ -1,3 +1,11 @@
+December 17, 2008
+- Fixed: Thing_ProjectileIntercept broke slightly when converted to the new
+  vector math routines (almost two years ago!) because the original code
+  multiplied down columns of the rotation matrix, but the new code multiplies
+  across rows of the matrix instead. This is remedied by flipping the matrix
+  across the x=y axis by reversing the sign of the sine value passed to the
+  matrix constructor.
+
 December 15, 2008
 - Fixed: Autoloading a game (e.g. respawning after dying in single player)
   during demo playback prematurely ended demo control of the player.
diff --git a/src/p_things.cpp b/src/p_things.cpp
index dc4690da1..b8273db57 100644
--- a/src/p_things.cpp
+++ b/src/p_things.cpp
@@ -173,7 +173,7 @@ bool P_Thing_Move (int tid, AActor *source, int mapspot, bool fog)
 	return false;
 }
 
-bool P_Thing_Projectile (int tid, AActor *source, int type, const char * type_name, angle_t angle,
+bool P_Thing_Projectile (int tid, AActor *source, int type, const char *type_name, angle_t angle,
 	fixed_t speed, fixed_t vspeed, int dest, AActor *forcedest, int gravity, int newtid,
 	bool leadTarget)
 {
@@ -288,7 +288,7 @@ bool P_Thing_Projectile (int tid, AActor *source, int type, const char * type_na
 							double ydotx = -aim | tvel;
 							double a = acos (clamp (ydotx / targspeed / dist, -1.0, 1.0));
 							double multiplier = double(pr_leadtarget.Random2())*0.1/255+1.1;
-							double sinb = clamp (targspeed*multiplier * sin(a) / fspeed, -1.0, 1.0);
+							double sinb = -clamp (targspeed*multiplier * sin(a) / fspeed, -1.0, 1.0);
 
 							// Use the cross product of two of the triangle's sides to get a
 							// rotation vector.