My sister has a lean on her house. Its really inconvenient for her.
In the above sentence what I really meant to say was, my sister has a lien on her house, i.e. a creditor is being nasty.
And I should have written It's, because that is the short form for It is. It is really inconvenient for her.
Its without the apostrophe is the possessive version of It: Its error [the error that belonged to it] caused me lost business.
The thing about Spell Check errors is that they can create some humorous - or even unfortunate - sentences. And they can cause you lost business, because they make you look unprofessional. Think of the following:
At the wild party, the celebrant drank too much, then fell asleep with his head in the desert.
The correct version is:
At the wild party, the celebrator drank too much, then fell asleep with his head in the dessert.
A celebrant is an officiating priest, not one who celebrates (although some priests might argue me on that point). And the desert is a barren land, not the delicious end to a meal.
The moral of the story? If you can't say what you mean, don't expect Spell Check to read your mind.
(And if you invite priests to your party in the desert, don't let them drink...)