I had the latter happen to me and so have several other people here on HN. LinkedIn ended up sending an invite to everyone I had ever emailed. It was catastrophically embarrassing and caused a lot of grief.
Not to detract from the main point of this thread (which I agree with) but do you mind sharing why you write "catastrophically" embarrassing? And caused 'a lot of grief'?
I'd never think twice about getting a linkedin invite email from anyone who for any reason has ever emailed me (ever). It's obviously automatic... I really can't think of an exception...
There are more email addresses lingering inside your email account than you probably realize. I just did a search for "LinkedIn sorry" to find all the apologies I sent out. 38 of them. All to complete and total strangers. When it first started happening I did some digging and found the connection for a lot of these people is we were on a common mailing list.
Not to mention the more obvious ones like ex-girlfriends, ex-bosses (including one I am not on good terms with), companies that I applied to and did not end up working for, etc. This is my gmail account, it has every email I've ever sent in the past 9 years. There's a lot of stuff in there.
EDIT: I don't know if LinkedIn still does this, but at the time LinkedIn would send reminder emails for any ignored invites. Which just compounded the problem.
If that is the worst thing that happens to you, you should be thanking LinkedIn for teaching you a valuable lesson about typing your email password into random forms on the Internet (or sharing passwords).
I'd never think twice about getting a linkedin invite email from anyone who for any reason has ever emailed me (ever). It's obviously automatic... I really can't think of an exception...