A few things:
1. Usually when you're helping people at the till, the customary thing to do is put your personal call on hold until the transaction is complete. Personally, I'm sure no one wanted to hear about so-and-so dumping this person, and how said person is scheming to win him back, yadda yadda yadda.
2. When I bring my purchases (in this case, a single bottle of water) to the counter, please do not act all exasperated and bent out of shape when I ask for a pack of cigarettes AFTER you present me with my total. You were too busy blabbing to your friend on the phone to hear me ask for them. Twice.
3. Do not merely thrust your finger at the keypad for the Interac machine (all the while still blabbing to your friend) to tell me to punch in my PIN. And don't act all snarky and bitchy (while still yapping to your friend) when I remind you that I also have a gas purchase.
4. My cousin had been in your establishment 10 minutes earlier, and she made mention that all you did was talk on the phone with your friend, doing the bare minimum to help out your customers.
5. Do not -- I repeat, do NOT -- use me as an excuse when some asshole decided to take off with a tank of gas without paying. For you to proclaim that you were "soooo busy" with my transaction and that I took up all of your concentration is complete and total bullshit. The only thing you were concentrating on was your telephone call, and you didn't put the receiver down until you realized someone took off with a tank of gas.
I have your name, I have the time/date I was there, I have my receipt. Hell, I even have the name of the friend you were talking to for the entire transaction. I won't make mention of this infraction to your boss (ie: calling him/her up and merely complaining about you and your lack of service), but if you dare try to use me as a scapegoat when your boss lowers the boom on you for allowing someone to take off with a tank of gas, you're in for a lot of grief. I don't care that the cameras can get a licence plate; if you even try to use my transaction as an excuse, your manager will get the full story from me in great detail.
Asking me for my name and number as a witness to the petrol thievery was a big mistake on your part.