[eru]

> Nothing about that language prevents code being full of bugs, [...]

Certain classes of bugs are impossible. You can still introduce all the possible bugs.

> Certainly, a competent Haskell programmer [..] will not introduce hidden side-effects.

I know this was meant as sarcasm, but even an incompetent Haskeller can not introduce hidden side-effects.

[PeterisP]

A particular side-effect that an incompetent Haskeller can (and will, multiple times) introduce is wiriting code that gives correct results and is seemingly correct, but will suddenly run out of memory on larger (but still reasonable-sized) data due to unexpected laziness.

[mightybyte]

In practice this doesn't happen very often. Laziness is unique to Haskell, but the phenomenon that bad programmers can introduce subtle bugs is most definitely not.