WY>Mechanics is important.

WY>That's all there is to it. Good mechanics is what makes the story readable.
WY>You don't have that, you are screwed no matter how good the plotline really
is.

While good mechanics is no doubt a necessity, one can
probably ignore a few errors here and there, especially
if the actual meaning is still highly evident and the mistake
isn't so glaring that it causes the reader a headache.

Too much, of course, will make the text a hassle to read,
but there is hardly a case where this happens. Most of us
here, even the newbies, can already write with a good
degree of competence.

Since what we do here is for fun and is not actual professional
publishing, I don't think that it's too unreasonable to have
a few mistakes here and there. Especially in the drafts,
whose real purpose it to get you writing as much of your
ideas as possible while they're still in your mind. You can
always correct them later.

Preferably, you can correct them after writing everything
down so that pre-reading is easier (from alpha to beta),
though it's always a plus if you can write as cleanly as
possibile the first run.

Unless you're a really obsessive-compulsive perfectionist, 
it takes the fun out of reading and writing to scrutinize another's 
work too heavily. You can probably let slide a few errors or two,
as long as they don't get in the way of understanding
the story. Besides, RPM's work wasn't badly written enough
to merit confusion in its reading. No harm done.

Just my two bits worth...