mało ważny, nieistotny, błahy, drobny

Boring: trivial, petty

• To fire somebody for that kind of petty mistake is outrageous.

Use: paltry or picayune or trifling

• To fire someone for that kind of picayune mistake is outrageous.
• trifling in a song:
• paltry in a song:

Not that picayune is informal American English. For Ngram of picayune, trifling, paltry, trivial, and petty click here.