diff --git a/src/main/java/catering/order/OrderController.java b/src/main/java/catering/order/OrderController.java
index 2ea4789..03d018f 100644
--- a/src/main/java/catering/order/OrderController.java
+++ b/src/main/java/catering/order/OrderController.java
@@ -157,23 +157,27 @@ public class OrderController {
 	@PostMapping("/event/addEmployee")
 	@PreAuthorize("hasRole('CUSTOMER')")
 	String addEmployeeToCart(@RequestParam("sid") long employeeId, @ModelAttribute("event") CustomCart cart) {
-		Employee employee = staffManagement.findById(employeeId).get();
+		Optional<Employee> employee = staffManagement.findById(employeeId);
 
-		if (cart.getStaff().contains(employee)) {
+		return employee.map(e -> {
+			if (cart.getStaff().contains(e)) {
+				return "redirect:/event";
+			}
+
+			cart.addEmployee(e);
 			return "redirect:/event";
-		}
-
-		cart.addEmployee(employee);
-		return "redirect:/event";
+		}).orElse("order-error");
 	}
 
 	@PostMapping("/event/removeEmployee")
 	@PreAuthorize("hasRole('CUSTOMER')")
 	String removeEmployeeFromCart(@RequestParam("sid") long employeeId, @ModelAttribute("event") CustomCart cart) {
-		Employee employee = staffManagement.findById(employeeId).get();
-		cart.removeEmployee(employee);
+		Optional<Employee> employee = staffManagement.findById(employeeId);
 
-		return "redirect:/event";
+		return employee.map(e -> {
+			cart.removeEmployee(e);
+			return "redirect:/event";
+		}).orElse("order-error");
 	}
 
 	@PostMapping("/allOrders/remove")
@@ -192,37 +196,40 @@ public class OrderController {
 
 	@PostMapping("/event/addProduct")
 	@PreAuthorize("hasRole('CUSTOMER')")
-	public String addProduct(@RequestParam("pid") Product product,
+	public String addProduct(@RequestParam("pid") Optional<Product> product,
 			@RequestParam("number") int number,
 			@ModelAttribute("event") CustomCart cart) {
-		// check if product is suitable
-		if (product.getCategories().stream().noneMatch(c -> c.equals(cart.getOrderType().toString()))) {
+
+		return product.map(p -> {
+			// check if product is suitable
+			if (p.getCategories().stream().noneMatch(c -> c.equals(cart.getOrderType().toString()))) {
+				return "redirect:/event";
+			}
+
+			Quantity amount = p.createQuantity(number > 0 ? number : 1);
+			Quantity cartQuantity = cart.getQuantity(p);
+			Quantity available;
+
+			if (p instanceof Rentable rentable) {
+				available = findFreeAmountInInterval(
+					rentable,
+					cart.getStart(),
+					cart.getFinish(),
+					inventory,
+					customOrderRepository);
+			} else {
+				available = inventory.findByProduct(p).get().getQuantity();
+			}
+
+			// check for possible miss-inputs
+			if (amount.add(cartQuantity).isGreaterThan(available)) {
+				cart.addOrUpdateItem(p, cartQuantity.negate().add(available));
+			} else {
+				cart.addOrUpdateItem(p, amount);
+			}
+
 			return "redirect:/event";
-		}
-
-		Quantity amount = product.createQuantity(number > 0 ? number : 1);
-		Quantity cartQuantity = cart.getQuantity(product);
-		Quantity available;
-
-		if (product instanceof Rentable rentable) {
-			available = findFreeAmountInInterval(
-				rentable,
-				cart.getStart(),
-				cart.getFinish(),
-				inventory,
-				customOrderRepository);
-		} else {
-			available = inventory.findByProduct(product).get().getQuantity();
-		}
-
-		// check for possible miss-inputs
-		if (amount.add(cartQuantity).isGreaterThan(available)) {
-			cart.addOrUpdateItem(product, cartQuantity.negate().add(available));
-		} else {
-			cart.addOrUpdateItem(product, amount);
-		}
-
-		return "redirect:/event";
+		}).orElse("order-error");
 	}
 
 	@PostMapping("/event/removeProduct")
@@ -264,6 +271,44 @@ public class OrderController {
 		}
 
 		return userAccount.map(account -> {
+			for (CartItem item : cart) {
+				// check if items in cart are still in inventory
+				if (inventory.findByProduct(item.getProduct()).isEmpty()) {
+					cart.clear();
+					return "order-error";
+				} else {
+					// check if there are still enough items left
+					Quantity invQuantity = inventory.findByProduct(item.getProduct()).get().getQuantity();
+					if (item.getQuantity().isGreaterThan(invQuantity)) {
+						cart.clear();
+						return "order-error";
+					}
+
+					// check availability for rentable
+					if (item.getProduct() instanceof Rentable rentable) {
+						Quantity available = findFreeAmountInInterval(
+							rentable,
+							cart.getStart(),
+							cart.getFinish(),
+							inventory,
+							customOrderRepository);
+
+						if (item.getQuantity().isGreaterThan(available)) {
+							cart.clear();
+							return "order-error";
+						}
+					}
+				}
+			}
+
+			// check if employees are still available
+			for (Employee employee : cart.getStaff()) {
+				if (staffManagement.findById(employee.getId()).isEmpty()) {
+					cart.clear();
+					return "order-error";
+				}
+			}
+
 			CustomOrder myOrder = new CustomOrder(account.getId(), cart);
 			cart.addItemsTo(myOrder);
 			cart.addStaffTo(myOrder);
diff --git a/src/main/resources/templates/order-error.html b/src/main/resources/templates/order-error.html
new file mode 100644
index 0000000..61802c3
--- /dev/null
+++ b/src/main/resources/templates/order-error.html
@@ -0,0 +1,19 @@
+<!--/*-->
+SPDX-License-Identifier: Apache-2.0 AND AGPL-3.0-or-later
+SPDX-FileCopyrightText: 2024 swt23w23
+<!--*/-->
+<!DOCTYPE html>
+<html xmlns:th="http://www.thymeleaf.org"
+	xmlns:sec="http://www.thymeleaf.org/extras/spring-security"
+	xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout"
+	layout:decorate="~{layout.html(title='Es ist ein Fehler aufgetreten')}">
+
+<body>
+<div layout:fragment="content">
+	<p>
+		Es tut uns seeeeeeehr leid. Leider ist bei ihrer Bestellung etwas schief gelaufen.
+		Bitte probieren Sie es später erneut.
+	</p>
+</div>
+</body>
+</html>