Spectroscopy - Forbidden transitions appearing in spectrum

[Jalo]

Homework Statement

Why do supposedly forbidden transitions appear in experimental spectrums?

Homework Equations

The Attempt at a Solution

I'm really puzzled as to why they appear. Can it be due to transitions on higher energy levels that happen to be equal to the forbidden ones?

 

[mfb]

"Forbidden" rarely means completely impossible in spectroscopy. It is just highly suppressed.
What does "suddenly" mean here?

Random coincidences with unrelated transitions are possible, but unlikely if the resolution is good enough.