Update MixedOperand.md

docs/running_psalm/issues/MixedOperand.md

@@ -7,3 +7,19 @@ Emitted when Psalm cannot infer a type for an operand in any calculated expressi
 
 echo $_GET['foo'] + "hello";
 ```
+
+## Why it’s bad
+
+Mixed operands can have fatal consequences, e.g. here:
+
+```php
+<?php
+
+function foo(mixed $m) {
+    echo $m . 'bar';
+}
+
+class A {}
+
+foo(new A()); // triggers fatal error
+```