I've been reading a lot of medical journal articles this week (not as bad as it sounds) and finding embarrassing errors, like this:
"Mortal Combat, for instead, is embedded in violence."
(Once you shake off the icky grammar, you can see the author probably meant "instance")
Another, in a questionnaire:
"The think you're most likely to do in your leisure time."
(Pretty sure the author meant "thing")
In both cases, the authors are highly-educated, world-renown scientists. Apparently, they're also prone to the same stupid mistakes the rest of us make when we spend long hours in coffee-fueled typing sessions.
When the errors are few, or found on page 57 of an otherwise solid research paper, they're excusable, yes? Usually, yes.
When the error is on your website where you're selling swimming pools, pink tiaras, or vitamin supplements... you run the risk of losing the sale.
Get an editor, or a friend that reads v e r y c a r e f u l l y.