I work shifts: week 1 8am-5pm, the next 12pm-8.30pm. I get in 5-10 mins early to have a cuppa but I start on the dot, and am ready to leave on the dot.
That said, I'm a trainee vet nurse so when it's busy, I have stayed 1, 2, 3 hours later, and it's not uncommon to miss your break. You kind of have to, it's a close knit team and if I leave on time when everythings gone mental, the other nurses have a really hard time. My boss is quite good about it though, I can usually take holiday at short notice or leave early later that week and he'll help out if I need a favour. Like has been said, it works both ways.