This is why I have a 'before you hit send' checklist. My list is a page long and includes making sure I'm pasting my query in as plain text, checking genre since some queries might have the addition of 'commercial' or 'upmarket' paired with a more descriptive genre. I'm so "whatever" that also on my list is to make sure I've added a Ms. or Mr. and that a period is at the end of those and a comma after the agent's name because I know someone who sent out a query lacking those things (the period and comma).
I just added 'are you sending from the right email account?' because in a recent batch I sent out, one was sent from a joke email account I have, that having a funny name that is a play on words that I use for my twitter account.

Anyway, sorry you went through that 'ack!' feeling. The first time I queried, I'd sent one out with the wrong name on it. Haven't forgotten that feeling, one very similar to realizing I'd sent one out from the wrong account.
And hey, gckatz, is copypasta an autocorrect gem that is quite awesome or is that a slang phrase I hadn't had the pleasure of seeing yet?