Hello all,
To preface, I am a newb

Can someone explain to me how unsafe unconfirmed transactions are please? What is the vulnerability? I have very little experience in this arena, but it seems like some sites (luckybit) can take an unconfirmed transaction and let you bet it on a drop, and other sites require a similarly small transaction to be confirmed.
Why?
Technically an unconfirmed amount of btc can be spent again which would nullify the transaction. It would take a high level of skill and luck to pull it off. It isn't something you or I will ever see done successfully but there is always a chance. Some businesses do it because the odds are low and you might choose them over someone else if you don't want to wait for confirmations.
Edited for spelling error