After hypo a cat will often be more sensitive to insulin. So, great care needs to be taken with the dosage (which will need to be
reduced), and it will also be important to monitor the blood glucose to check that the reduced dose is still OK for the cat.
A skipped shot isn't the end of the world. Many of us have given 'fur shots' just by accident. (And a few of us have just forgotten to give the shot on occasion!

)
Any high numbers you see later on will
probably be a combination of a bounce (body releasing stored glucose to counteract the low number (this can take up to several days to clear)); the insulin 'depot' running low; and any high carb food you've fed. Basically though, you just have to ride it out... Things will settle into their groove again pretty soon.
Eliz