You need to turn off popups or open in new tab to see these otherwise they get snippet'd out.
Edit a recurring contribution:
Notice: Undefined index: transaction in CRM_Contribute_Form_AbstractEditPayment->assignPaymentInfoBlock() (line 721 of .../CRM/Contribute/Form/AbstractEditPayment.php).
Save a recurring contribution after enabling tax and invoicing but before setting up the tax account and assigning to the financial type:
Notice: Undefined index: tax_amount in CRM_Contribute_Form_Contribution_Confirm::processFormContribution() (line 957 of .../CRM/Contribute/Form/Contribution/Confirm.php
https://github.com/civicrm/civicrm-core/pull/18006 (not a typo - same PR as above)
After adding gender to profile:
Notice: Undefined offset: 2 in CRM_UF_Form_Field->postProcess() (line 533 of /.../CRM/UF/Form/Field.php).
After saving dummy SMS provider:
This is sort of both the fault of the provider and core. Core should handle if it's missing, but in general providers would specify the service url and it's a required field on the form, e.g. for twilio.
Notice: Undefined property: io_3sd_dummysms::$_apiURL in CRM_SMS_Form_Provider->setDefaultValues() (line 117 of .../CRM/SMS/Form/Provider.php).
Copy To Case and its relatives when activity subject is blank:
(You can't see this in the UI since you can't get here without a popup, you need to look in drupal watchdog)
Notice: Undefined index: subject in CRM_Case_Form_ActivityToCase->setDefaultValues() (line 63 of .../CRM/Case/Form/ActivityToCase.php).
After adding a payment processor:
Notice: Undefined property: CRM_Admin_Form_PaymentProcessor::$_testID in CRM_Admin_Form_PaymentProcessor->updatePaymentProcessor() (line 483 of .../CRM/Admin/Form/PaymentProcessor.php).