The last thing you want to do is send the wrong person an email intended for someone else. Depending on the content, this could be a relationship-killing mistake, but in any case it looks unprofessional and might make people wonder about you. As of now, Gmail's autocomplete might be very untrustworthy now.
TechCrunch reports on this bug, which a Google spokesperson has acknowledged as an issue the company is currently investigating. TechCrunch has rounded up a few tweets describing the bug from prominent startup folk and venture capitalist, including this one from Mac Andreessen:
The incipient malevolent human-murdering AI inside GMail just autocompleted an address I've emailed once over one I've emailed 10,000 times.
It's never a good idea to trust autocomplete (as everyone who's ever typed an email on a tablet or smartphone has probably learned through experience), but now is a good idea to be extra careful when trusting Gmail, until the issue/bug is fixed.