I'm not sure whether others find this as frustrating (or even know that it happens!) but we regularly find in our own portal and with clients that if you send a sales email using the Outlook or Gmail integration, 'log to CRM' and CC someone who is not a contact in HubSpot, they get created as a contact without you knowing.
I assume this is part of an automatic feature which would help you out in a situation where you're emailing a contact who doesn't exist in HubSpot.
My issue is that this is not good when you have CC'd an external supplier / partner or even other members of your clients team who need to be involved on a CC loop but are not a contact. Your client may have even emailed a group of people on a subject and you are simply replying.
Why is that an issue?
A) the number of contacts rises (and could impact your paid contact tiers) and there is no notification of which contacts have been added by accident this way - we have had to create a very confusing list to filter these people but I still worry some get missed.
B) They are automatically also in the marketing side of HubSpot, which is generally a good feature, but in this case difficult to manage.
With GDPR coming into effect soon I hope this is a feature which can be looked at to stop this happening or even to provide a manual check (even a tick box to say 'add these new contacts to the CRM?' so that there is a prompt to then go and mark them as not opted in).