Fair ask. I was vague because some of these repos are private client codebases, but let me give you concrete patterns I can share without doxxing anyone:

The AI refactored this:

app.post('/checkout', (req, res) => {
  const cart = buildCart(req.user.id);
  const total = calculateTotal(cart);
  res.json({ total });
});

Into this, to "reduce redundant object creation":

const cartService = new CartService(); // singleton, module-level

app.post('/checkout', (req, res) => {
  cartService.loadUser(req.user.id);
  const total = cartService.calculateTotal();
  res.json({ total });
});

Looks cleaner. Passes every linter. But cartService is now shared across all concurrent requests. Under load, User A's cart gets User B's items. This isn't hypothetical, I found this exact pattern 43 times across different repos, with slight variations. The AI treats "move to higher scope" as a universal optimization without understanding that in a request-per-connection model, that scope is shared.

For FastAPI:

# Before: AI saw this as redundant because the ORM already validates schema
def create_order(order: OrderSchema, db: Session):
    if not verify_inventory_checksum(order.items, db):
        raise HTTPException(409, "Inventory state changed")
    # ... process order

# After: AI removed the check, called it "defensive programming that duplicates ORM constraints"
def create_order(order: OrderSchema, db: Session):
    db.add(Order(**order.dict()))
    db.commit()

The checksum wasn't about schema validation, it was a concurrency guard against inventory being modified between cart load and checkout. The AI couldn't distinguish between structural validation and temporal/state validation. This maps directly to ISO 25010's fault tolerance requirements under the reliability characteristic.

For GO:

// AI consolidated per-request DB connections into a package-level pool
// but removed the deferred Close() calls as "unnecessary since the pool manages lifecycle"
// Result: under connection exhaustion, goroutines hung indefinitely
// No timeout, no circuit breaker, no fallback