A stone thrown out of a moving vehicle on Marroway, Weston Turville, smashed through a windscreen and hit a 66-year-old woman in the face.
Days earlier a stone was thrown out of a moving vehicle on the same road, but nobody was injured.
The first stone was thrown at around 7pm on Tuesday March 14, while the second was thrown between 9pm and 9.30pm on Thursday March 16.
The lady who was hit in the face by the stone did not sustain any injuries or require hospital treatment.
Police are appealing for information in relation to the assault and criminal damage.
PC Joanna Smith of the Neighbourhood policing team based at Wendover police station said: “I would like to appeal for any witnesses to this incident to contact me.
"If you have any information please contact me or PC Thomas Piercy via the Thames Valley Police enquiry centre 101.
“If you don’t want to speak directly to the police you can contact the independent Charity Crimestoppers anonymously on 0800 555 111. No personal details are taken, information is not traced or recorded and you will not go to court.”