We tend to lose Rs after syllable initial bilabials and labio-dentals. Witness Febyuary, libary, infastructure, and speak/speech.

Re: on vs. at the weekend. Prepositions tend to be slippery. On accident is becoming increasingly popular among younger speakers. Or in line vs. on line, the latter being common in New York City for the former (not in reference to being logged on to a computer network).