I've had a homeless guy get angry and yell at me because I didn't let him bum a cigarette. I don't smoke and have never carried cigarettes before. It looked like he was just about to stab me over a damn cigarette I didn't even have. These types of interactions are far too frequent. It invades almost every single facet of every-day life.
Around here it's also very common at grocery stores for homeless to grab your shopping cart and demand to be paid for putting your cart up. They like to target women especially if they have kids with them because they're much more likely to comply. If you deny them they're not all that understanding. Growing up in the city I've had my fair share of altercations just doing normal every day things. Honestly it's like mosquitos.