Window open in one group but not the other meaning they also were exposed to traffic noise.
One member of the group coughing throughout the encoding phase for one group bit not the other.
Researcher is sloppy and forgets to start the timer straight away, for one group but not the other when they are in the encoding phase meaning they have a little bit longer to look at the words than the other group.
The extract given out to one group in the distractor task is not very well photocopied making it a little smudgy and harder to read, making the task trickier for one group compared with the other, potentially leaving them in a different state of mind before the recall phase compared with the other group.
In one group the researchers are whispering to each other about something whilst the Pps are doing the recall task, but in the other group they are silent.
The room is very hot in one condition but not the other.