[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.