diff --git a/redbot/cogs/mod/mutes.py b/redbot/cogs/mod/mutes.py
index 5ee6126cc..ee903c7ce 100644
--- a/redbot/cogs/mod/mutes.py
+++ b/redbot/cogs/mod/mutes.py
@@ -22,6 +22,8 @@ mute_unmute_issues = {
         "permission and the user I'm muting must be "
         "lower than myself in the role hierarchy."
     ),
+    "left_guild": _("The user has left the server while applying an overwrite."),
+    "unknown_channel": _("The channel I tried to mute the user in isn't found."),
 }
 _ = T_
 
@@ -422,6 +424,11 @@ class MuteMixin(MixinMeta):
             await channel.set_permissions(user, overwrite=overwrites, reason=reason)
         except discord.Forbidden:
             return False, _(mute_unmute_issues["permissions_issue"])
+        except discord.NotFound as e:
+            if e.code == 10003:
+                return False, _(mute_unmute_issues["unknown_channel"])
+            elif e.code == 10009:
+                return False, _(mute_unmute_issues["left_guild"])
         else:
             await self.settings.member(user).set_raw(
                 "perms_cache", str(channel.id), value=old_overs
@@ -460,6 +467,11 @@ class MuteMixin(MixinMeta):
                 await channel.set_permissions(user, overwrite=overwrites, reason=reason)
         except discord.Forbidden:
             return False, _(mute_unmute_issues["permissions_issue"])
+        except discord.NotFound as e:
+            if e.code == 10003:
+                return False, _(mute_unmute_issues["unknown_channel"])
+            elif e.code == 10009:
+                return False, _(mute_unmute_issues["left_guild"])
         else:
             await self.settings.member(user).clear_raw("perms_cache", str(channel.id))
             return True, None