Salesforce Winter’25 Release Notes | Development – Apex

Starting from API version 62.0, modifying elements of a set while iterating the set in a for or foreach() loop throws an exception: System.FinalException: Cannot modify a collection while it is being iterated. In the previous API versions, modifications to sets while iterating were sometimes allowed and generated unexpected results

Example: Below code removes elements while iterating the set, which throws an exception: System.FinalException

Set<String> set_string = new Set<String>{'one', 'two', 'three'};
for (String str : set_string) {
  System.debug(str);
  set_string.remove(str);
 
  System.debug(set_string.contains(str));
}
System.debug(set_string);