### Clue: Mistakes note on unknown vessels, missing cable's header (3-3)
### Answer: MIX-UPS 
 ### Explanation: Definition: Mistakes
 MI(syllable for one of the notes on the musical scale) **plus**(on) X(in maths, symbol for an unknown quantity) + "cups"(vessels;containers) **minus**(missing) **the 1st letter of**(…'s header) "cable".