diff --git a/src/exper.c b/src/exper.c
index d55f9abab..1fe07dcd1 100644
--- a/src/exper.c
+++ b/src/exper.c
@@ -321,8 +321,8 @@ pluslvl(
         u.mh += hpinc;
     }
     hpinc = newhp();
-    setuhpmax(u.uhpmax + hpinc);
     u.uhp += hpinc;
+    setuhpmax(u.uhpmax + hpinc); /* will lower u.uhp if it exceeds uhpmax */
 
     /* increase spell power/energy points */
     eninc = newpw();
diff --git a/src/hack.c b/src/hack.c
index 2b9ca664c..1e249a68c 100644
--- a/src/hack.c
+++ b/src/hack.c
@@ -4039,10 +4039,15 @@ maybe_wail(void)
 int
 saving_grace(int dmg)
 {
-    if (!u.usaving_grace && (u.uhp <= dmg)
-        && (u.uhp * 100 / u.uhpmax) > 90) {
+    if (dmg < 0) {
+        impossible("saving_grace check for negative damage? (%d)", dmg);
+        return 0;
+    }
+
+    if (!u.usaving_grace && dmg >= u.uhp && (u.uhp * 100 / u.uhpmax) > 90) {
+        /* note: this could reduce dmg to 0 if u.uhpmax==1 */
         dmg = u.uhp - 1;
-        u.usaving_grace = TRUE; /* used up */
+        u.usaving_grace = 1; /* used up */
         end_running(TRUE);
         if (u.usleep)
             unmul("Suddenly you wake up!");
diff --git a/src/mcastu.c b/src/mcastu.c
index 68a8a8119..90f9bd9c3 100644
--- a/src/mcastu.c
+++ b/src/mcastu.c
@@ -342,14 +342,12 @@ castmu(
         break;
     case AD_SPEL: /* wizard spell */
     case AD_CLRC: /* clerical spell */
-    {
         if (mattk->adtyp == AD_SPEL)
             cast_wizard_spell(mtmp, dmg, spellnum);
         else
             cast_cleric_spell(mtmp, dmg, spellnum);
         dmg = 0; /* done by the spell casting functions */
         break;
-    }
     } /* switch */
     if (dmg)
         mdamageu(mtmp, dmg);
@@ -441,6 +439,11 @@ death_inflicted_by(
 staticfn void
 cast_wizard_spell(struct monst *mtmp, int dmg, int spellnum)
 {
+    if (dmg < 0) {
+        impossible("monster cast wizard spell (%d) with negative dmg (%d)?",
+                   spellnum, dmg);
+        return;
+    }
     if (dmg == 0 && !is_undirected_spell(AD_SPEL, spellnum)) {
         impossible("cast directed wizard spell (%d) with dmg=0?", spellnum);
         return;
@@ -620,6 +623,12 @@ staticfn void
 cast_cleric_spell(struct monst *mtmp, int dmg, int spellnum)
 {
     int orig_dmg = 0;
+
+    if (dmg < 0) {
+        impossible("monster cast cleric spell (%d) with negative dmg (%d)?",
+                   spellnum, dmg);
+        return;
+    }
     if (dmg == 0 && !is_undirected_spell(AD_CLRC, spellnum)) {
         impossible("cast directed cleric spell (%d) with dmg=0?", spellnum);
         return;
diff --git a/src/mhitu.c b/src/mhitu.c
index b7f338294..33e738189 100644
--- a/src/mhitu.c
+++ b/src/mhitu.c
@@ -1211,7 +1211,7 @@ hitmu(struct monst *mtmp, struct attack *mattk)
             else if (*hpmax_p > lowerlimit)
                 *hpmax_p = lowerlimit;
             /* else unlikely...
-             * already at or below minimum threshold; do nothing */
+             * already at or below minimum threshold, do nothing to hpmax */
             disp.botl = TRUE;
         }
 
@@ -1846,16 +1846,27 @@ gazemu(struct monst *mtmp, struct attack *mattk)
 void
 mdamageu(struct monst *mtmp, int n)
 {
+    if (n < 0) {
+        impossible("mdamageu for negative damage? (%d)", n);
+        n = 0;
+    }
+
     disp.botl = TRUE;
     if (Upolyd) {
         u.mh -= n;
         showdamage(n);
+        /* caller might have reduced mhmax before calling mdamageu() */
+        if (u.mh > u.mhmax)
+            u.mh = u.mhmax;
         if (u.mh < 1)
             rehumanize();
     } else {
         n = saving_grace(n);
         u.uhp -= n;
         showdamage(n);
+        /* caller might have reduced uhpmax before calling mdamageu() */
+        if (u.uhp > u.uhpmax)
+            u.uhp = u.uhpmax;
         if (u.uhp < 1)
             done_in_by(mtmp, DIED);
     }
@@ -2422,10 +2433,10 @@ passiveum(
                 break;
             }
             pline("%s is suddenly very cold!", Monnam(mtmp));
-            u.mh += tmp / 2;
+            u.mh += (tmp + rn2(2)) / 2;
             if (u.mhmax < u.mh)
                 u.mhmax = u.mh;
-            if (u.mhmax > ((gy.youmonst.data->mlevel + 1) * 8))
+            if (u.mhmax > (((int) gy.youmonst.data->mlevel + 1) * 8))
                 (void) split_mon(&gy.youmonst, mtmp);
             break;
         case AD_STUN: /* Yellow mold */
diff --git a/src/uhitm.c b/src/uhitm.c
index 307883235..bda3557f0 100644
--- a/src/uhitm.c
+++ b/src/uhitm.c
@@ -5906,11 +5906,11 @@ passive(
                 You("are suddenly very cold!");
                 mdamageu(mon, tmp);
                 /* monster gets stronger with your heat! */
-                mon->mhp += tmp / 2;
+                mon->mhp += (tmp + rn2(2)) / 2;
                 if (mon->mhpmax < mon->mhp)
                     mon->mhpmax = mon->mhp;
                 /* at a certain point, the monster will reproduce! */
-                if (mon->mhpmax > ((int) (mon->m_lev + 1) * 8))
+                if (mon->mhpmax > (((int) mon->m_lev) + 1) * 8)
                     (void) split_mon(mon, &gy.youmonst);
             }
             break;