diff --git a/Source/Glide64/Debugger.cpp b/Source/Glide64/Debugger.cpp
index c0ac22f38..8775524d4 100644
--- a/Source/Glide64/Debugger.cpp
+++ b/Source/Glide64/Debugger.cpp
@@ -41,6 +41,15 @@
 #include "Util.h"
 #include "Debugger.h"
 
+/*
+ * required to include OpenGL library without errors
+ * Dependency on OpenGL in this module is limited to just `glGetError`.
+ */
+#ifdef _WIN32
+#include <windows.h>
+#endif
+#include <GL/gl.h>
+
 GLIDE64_DEBUGGER _debugger;
 
 #define SX(x) ((x)*rdp.scale_1024)
@@ -1018,3 +1027,52 @@ void output (float x, float y, int scale, const char *fmt, ...)
     x+=8;
   }
 }
+
+static const char * GL_errors[7 + 1] = {
+    "GL_NO_ERROR", /* "There is no current error." */
+    "GL_INVALID_ENUM", /* "Invalid parameter." */
+    "GL_INVALID_VALUE", /* "Invalid enum parameter value." */
+    "GL_INVALID_OPERATION", /* "Illegal call." */
+    "GL_STACK_OVERFLOW",
+    "GL_STACK_UNDERFLOW",
+    "GL_OUT_OF_MEMORY", /* "Unable to allocate memory." */
+
+    "GL_UNKNOWN_ERROR" /* ??? */
+};
+
+int grDisplayGLError(const char* message)
+{
+    GLenum status;
+    unsigned int error_index;
+    int failure;
+
+    status = glGetError();
+    failure = 1;
+
+    if (status == GL_NO_ERROR)
+        error_index = failure = 0;
+    else
+        error_index =
+            (status < GL_INVALID_ENUM) /* to avoid underflow when subtracting */
+          ? ( 7 ) /* our own, made-up "GL_UNKNOWN_ERROR" error */
+          : (status - GL_INVALID_ENUM) + 1;
+
+    if (error_index > 7)
+        error_index = 7;
+
+#if !0
+/*
+ * In most cases, we don't want to spam the screen to repeatedly say that
+ * there were no OpenGL errors yet, though sometimes one may need verbosity.
+ */
+    if (failure == 0)
+        return (failure);
+#endif
+
+#ifdef _WIN32
+    MessageBoxA(NULL, message, GL_errors[error_index], MB_ICONERROR);
+#else
+    fprintf(stderr, "%s\n%s\n\n", GL_errors[error_index], message);
+#endif
+    return (failure);
+}
diff --git a/Source/Glide64/Debugger.h b/Source/Glide64/Debugger.h
index cc6b7444c..f304d4eea 100644
--- a/Source/Glide64/Debugger.h
+++ b/Source/Glide64/Debugger.h
@@ -135,3 +135,5 @@ void debug_cacheviewer ();
 void debug_mouse ();
 void debug_keys ();
 void output (float x, float y, int scale, const char *fmt, ...);
+
+extern int grDisplayGLError( const char * message );
diff --git a/Source/Glide64/Main.cpp b/Source/Glide64/Main.cpp
index 2b3dfc2ed..ea4f83e02 100644
--- a/Source/Glide64/Main.cpp
+++ b/Source/Glide64/Main.cpp
@@ -1890,6 +1890,10 @@ void CALL UpdateScreen (void)
   if (fullscreen && (*gfx.VI_ORIGIN_REG  > width))
     update_screen_count++;
 
+#if defined(_DEBUG) || 0
+  grDisplayGLError("UpdateScreen");
+#endif
+
 #ifdef FPS
   // vertical interrupt has occurred, increment counter
   vi_count ++;
diff --git a/Source/Glitch64/main.cpp b/Source/Glitch64/main.cpp
index 27055002a..a3bfaa9ef 100644
--- a/Source/Glitch64/main.cpp
+++ b/Source/Glitch64/main.cpp
@@ -2115,6 +2115,10 @@ grBufferSwap( FxU32 swap_interval )
   for (i = 0; i < nb_fb; i++)
     fbs[i].buff_clear = 1;
 
+#ifdef _DEBUG
+  grFinish();
+#endif
+
   // VP debugging
 #ifdef VPDEBUG
   dump_stop();
@@ -2572,13 +2576,13 @@ grErrorSetCallback( GrErrorCallbackFnc_t /*fnc*/ )
 FX_ENTRY void FX_CALL
 grFinish(void)
 {
-  display_warning("grFinish");
+  glFinish();
 }
 
 FX_ENTRY void FX_CALL
 grFlush(void)
 {
-  display_warning("grFlush");
+  glFlush();
 }
 
 FX_ENTRY void FX_CALL