Don't leave home without a spares & tool kit for both roadside repairs and building bikes when travelling.
A loose bar can end a race, as can cracking carbon cos you didn't use a torque wrench. A bike stand and handy travel pump can be godsends in small hotel rooms. Drink cage tools holders are best but they take up a cage while seat post loaded ones can shake off.
Bar tape unravels or can be slippery with wet swim hands. A rough number belt can chaffe or ride up and must have toggles. Gloves help with wetsuit tears.
Different mounts can be used for tech controls. So - what nifty gadgets have you hated or rated and why?