It is good habit to add a personal note when inviting someone to connect through LinkedIn to explain how you know the other person and why you want to connect. However, lately I noticed a small inconvenience with this approach:
I invited a few people with a personalised note and instead of the acceptation of the invitation I received a reply to my note.
It is no big problem, as I simply reply back and ask them to accept my invitation. Maybe LinkedIn could think of a 'Reply & Accept' button?