diff --git a/code/ai/aigoals.cpp b/code/ai/aigoals.cpp
index 9402b7f7b0b..9c0dbf50ddd 100644
--- a/code/ai/aigoals.cpp
+++ b/code/ai/aigoals.cpp
@@ -825,7 +825,8 @@ void ai_add_goal_sub_player(ai_goal_type type, ai_goal_mode mode, int submode, c
 // friendlies want to rearm at the same time.  The support ship forgets what it's doing and flies
 // off to repair somebody while still docked.  I reproduced this with retail, so it's not a bug in
 // my new docking code. :)
-int ai_goal_find_empty_slot( ai_goal *goals, int active_goal )
+// Note that this function is now where Purge_when_new_goal_added is checked to set the Purge flag.
+int ai_goal_allocate_slot( ai_goal *goals, int active_goal )
 {
 	int oldest_index = -1, first_empty_index = -1;
 
@@ -915,7 +916,7 @@ void ai_add_ship_goal_scripting(ai_goal_mode mode, int submode, int priority, co
 	int empty_index;
 	ai_goal *aigp;
 
-	empty_index = ai_goal_find_empty_slot(aip->goals, aip->active_goal);
+	empty_index = ai_goal_allocate_slot(aip->goals, aip->active_goal);
 	aigp = &aip->goals[empty_index];
 	ai_add_goal_sub_scripting(ai_goal_type::PLAYER_SHIP, mode, submode, priority, shipname, aigp, int_data, float_data);
 
@@ -939,7 +940,7 @@ void ai_add_ship_goal_player(ai_goal_type type, ai_goal_mode mode, int submode,
 	int empty_index;
 	ai_goal *aigp;
 
-	empty_index = ai_goal_find_empty_slot( aip->goals, aip->active_goal );
+	empty_index = ai_goal_allocate_slot( aip->goals, aip->active_goal );
 	aigp = &aip->goals[empty_index];
 	ai_add_goal_sub_player( type, mode, submode, shipname, aigp, int_data, float_data, lua_target );
 
@@ -973,7 +974,7 @@ void ai_add_wing_goal_player(ai_goal_type type, ai_goal_mode mode, int submode,
 	// add the sexpression index into the wing's list of goal sexpressions if
 	// there are more waves to come.  We use the same method here as when adding a goal to
 	// a ship -- find the first empty entry.  If none exists, take the oldest entry and replace it.
-	empty_index = ai_goal_find_empty_slot( wingp->ai_goals, -1 );
+	empty_index = ai_goal_allocate_slot( wingp->ai_goals, -1 );
 	ai_add_goal_sub_player( type, mode, submode, shipname, &wingp->ai_goals[empty_index], int_data, float_data, lua_target );
 }
 
@@ -1613,7 +1614,7 @@ void ai_add_ship_goal_sexp( int sexp, ai_goal_type type, ai_info *aip )
 {
 	int gindex;
 
-	gindex = ai_goal_find_empty_slot( aip->goals, aip->active_goal );
+	gindex = ai_goal_allocate_slot( aip->goals, aip->active_goal );
 	ai_add_goal_sub_sexp( sexp, type, aip, &aip->goals[gindex], Ships[aip->shipnum].ship_name );
 }
 
@@ -1637,7 +1638,7 @@ void ai_add_wing_goal_sexp(int sexp, ai_goal_type type, wing *wingp)
 	if ((wingp->num_waves - wingp->current_wave > 0) || Fred_running) {
 		int gindex;
 
-		gindex = ai_goal_find_empty_slot( wingp->ai_goals, -1 );
+		gindex = ai_goal_allocate_slot( wingp->ai_goals, -1 );
 		ai_add_goal_sub_sexp( sexp, type, nullptr, &wingp->ai_goals[gindex], wingp->name );
 	}
 }
@@ -1660,7 +1661,7 @@ void ai_add_goal_ship_internal( ai_info *aip, int goal_type, char *name, int  /*
 	Assertion(strcmp(name, Ships[aip->shipnum].ship_name) != 0, "The goals apply to the actor in ai_add_goal_ship_internal for ship %s, please report to the SCP!", name);
 
 	// find an empty slot to put this goal in.
-	gindex = ai_goal_find_empty_slot( aip->goals, aip->active_goal );
+	gindex = ai_goal_allocate_slot( aip->goals, aip->active_goal );
 	aigp = &(aip->goals[gindex]);
 	ai_goal_reset(aigp, true);
 
@@ -1789,7 +1790,7 @@ ai_achievability ai_mission_goal_achievable( int objnum, ai_goal *aigp )
 		if (!target_ship_entry || !target_ship_entry->has_shipp())
 			return ai_achievability::NOT_ACHIEVABLE;
 
-		// the override flag is now set in the calling function, ai_mission_goal_achievable
+		// the override flag is now set in the calling function, validate_mission_goals
 		return ai_achievability::ACHIEVABLE;
 	}
 
diff --git a/code/parse/sexp.cpp b/code/parse/sexp.cpp
index 3c3cda2c890..8982e41b8e9 100644
--- a/code/parse/sexp.cpp
+++ b/code/parse/sexp.cpp
@@ -2689,7 +2689,10 @@ int check_sexp_syntax(int node, int desired_return_type, int recursive, int *bad
 						}
 						break;
 				}
-				Assert(ship_node >= 0);
+				if (ship_node < 0) {
+					Warning(LOCATION, "Could not find ship node for operator %s!", Operators[op_index].text.c_str());
+					return SEXP_CHECK_INVALID_SHIP;
+				}
 
 				if (is_node_value_dynamic(ship_node)) {
 					const int dyn_val_check = check_dynamic_value_node_type(ship_node, true, false);
@@ -4034,7 +4037,10 @@ int check_sexp_syntax(int node, int desired_return_type, int recursive, int *bad
 				}
 
 				p_container = get_sexp_container(Sexp_nodes[node].text);
-				Assertion(p_container, "Attempt to use unknown container %s. Please report!", Sexp_nodes[node].text);
+				if (!p_container) {
+					Warning(LOCATION, "Attempt to use unknown container %s. Please report!", Sexp_nodes[node].text);
+					return SEXP_CHECK_TYPE_MISMATCH;
+				}
 
 				if ((desired_argument_type == OPF_LIST_CONTAINER_NAME && !p_container->is_list()) ||
 						(desired_argument_type == OPF_MAP_CONTAINER_NAME && !p_container->is_map())) {
@@ -4044,10 +4050,10 @@ int check_sexp_syntax(int node, int desired_return_type, int recursive, int *bad
 			}
 
 			case OPF_CONTAINER_VALUE:
-				Assertion(p_container,
-					"Attempt to check value arg for null container for SEXP operator %d at arg %d. Please report!",
-					op_const,
-					argnum);
+				if (!p_container) {
+					Warning(LOCATION, "Attempt to check value arg for null container for SEXP operator %s at arg %d. Please report!", Operators[op_index].text.c_str(), argnum);
+					return SEXP_CHECK_TYPE_MISMATCH;
+				}
 				z = check_container_value_data_type(op_const,
 					argnum,
 					p_container->type,
@@ -4063,9 +4069,10 @@ int check_sexp_syntax(int node, int desired_return_type, int recursive, int *bad
 				if (node_subtype == SEXP_ATOM_CONTAINER_NAME) {
 					// only list containers of strings or map containers with string keys are allowed
 					const auto *p_str_container = get_sexp_container(Sexp_nodes[node].text);
-					Assertion(p_str_container,
-						"Attempt to use unknown container %s. Please report!",
-						Sexp_nodes[node].text);
+					if (!p_str_container) {
+						Warning(LOCATION, "Attempt to use unknown container %s. Please report!", Sexp_nodes[node].text);
+						return SEXP_CHECK_TYPE_MISMATCH;
+					}
 
 					const auto &str_container = *p_str_container;
 					if (str_container.is_list() && none(str_container.type & ContainerType::STRING_DATA)) {