The device needing more power won’t get it, simple. Depending on what device it is, it will automatically throttle down so it needs less power, but obviously it will also deliver less performance while so throttled. And if the power is missing during a very sensitive part of a process so there’s no time to throttle down, your PC could blue screen or restart.
It’s very unlikely to suffer any long-term damage from this.
Even worse? Senior devs that are confidently incorrect but are trusted completely because they created an “amazing” VBA macro for Excel 97 once.