Why is using a simple static Boolean flag to prevent recursion dangerous when processing more than 200 records?

Triggers process records in chunks of 200. A static variable lives for the whole transaction, so after the first chunk sets hasRun = true, chunks 2, 3... in the same transaction see the flag and skip all logic, leaving records 201+ unprocessed (silently).

Fix: track processed record Ids in a static Set<Id> and process only Ids not yet in the set. For updates, you can also reprocess a record only if relevant fields changed.

public static Set<Id> processed = new Set<Id>();
for (Account a : Trigger.new) {
    if (processed.add(a.Id)) { /* process */ }
}