For example: when changing from 0500 to 7000 (VFR code): you might turn the first knob to a 7 (thus 7500), and at this time the second knob still is set to 5. This would momentarily have the transponder replying the hijack code (7500), which might lead to a lot more attention than desired. I would recommend to set the transponder in 'standby mode' before changing the codes. It will cause a temporarily loss of target info on the radar screen, but it's safer than having bells going off in ATC centers and F16s scramble for you because you accidentally had 7500 displayed on the radar screen.
Someone else's suggestion.
Where in the stack was the transponder? was it easily readable a quick look down under pressure a 5 could be seen as 0.
Or it could have been Sid's last thought
All ready got the coat on