Two national bloggers who cover Apple told The Oregonian it’s plausible the whole incident stemmed from a typo or random keystroke, but there’s no way to know for sure.

Portland; Portland Apple Store customer claims store employee typed homophobic slur on receipt. Updated: ; Aug. 12, 2014, 9:59 p.m. | Published: ; Aug. 12, 2014, 8:59 ...

According to a report by BetaBeat, Portland resident Adam Catanzarite found one such message typed onto an Apple Store receipt for some headphones he purchased. Under his name, it read “[email protected].” ...

Again, Apple can't be insisting to print-out the receipt, it's an online store! when one enters the information in the online store, it is just as visible as when your order is complete.