Defining the system
An energy account starts by defining a system and its surroundings. A body may gain energy because another body supplies it, without violating conservation. For an isolated system the total remains constant, so apparent gains or losses require examination of omitted interactions and energy forms.
Friction and internal energy
Kinetic and potential energy can exchange while their mechanical sum remains constant when dissipation is absent. Friction reduces mechanical energy but increases internal energy and other contributions. Tracking motion alone can therefore produce an incomplete account of a process that still conserves total energy.
References: [1]
Heat, work and the first law
The first law of thermodynamics relates changes in internal energy to heat transfer and work. With work done by the system taken as positive, the change is heat received minus work delivered. Heat and work describe transfers; internal energy describes a system state.
References: [2]
Mass differences in reactions
Relativistic accounting includes rest energy. The sum of product rest masses in a nuclear reaction can differ from the initial sum, with the difference reflected in kinetic energy and radiation. Conservation applies to the complete energy balance, rather than to rest mass considered separately.
References: [3]