How does the splicing enzyme recognize where to splice the introns?
There are two factors that involve the ability of enzymes to process RNA.
1) Structure see wikipedia
2) Binding affinitya

Let's take a look at the splicing process:


The active 'sites' (GU,A &amp; AG) need to be in spatial proximity (point one), and the enzyme needs to be able to bind there, aka forming hydrogen bonds with the nucleotides, which is mostly the case if:
a) the nucleotides are 'free' as in they are not paired with other nucleotides
b) it is energetically favorable for the enzyme to bind and do its work, which factors like temperature, and of course on the correct nucleotides

If there were some mutations, the splicing might not work.