Firstly, Welcome! I take it kitty's name is Silver? How old is he/she? I see on your SS that Silver is on antibiotics. Please know that infection can cause Blood Glucose to shoot way up, and when the medication kicks in, the BG can drop very quickly. Test as much as you are able to watch for quick drops.
As for the 500 today, that is completely normal. The quick drop kitty did yesterday (which might have gone quite low last night) caused kitty to "bounce" . Simply put, a bounce is a reaction by a body that is used to running with high BG being suddenly low. The liver panics and kicks out glucose, thinking" oh dear, we are in trouble here, we need sugar in case we have to fight or run!" It is extremely common, and takes a long time for the liver to be retrained and calm down as more and more normal levels are achieved.
I am concerned about Silver being started on 2 units while fighting infection though. That is a lot of insulin for a starting dose. Also, what are you feeding Silver? If you have changed to low carb wet food (and we highly recommend that you do), then I would immediately reduce the insulin dose to 1 unit
We say here "better high for a day than too low for a moment"
Please introduce us to yourself and your dear Silver, and we look forward to helping you while we get to know you. This community has more combined knowledge about feline diabetes than many vets, so know you have landed in the right place. Had I not had this group, I am sure I would have lost my Samantha, as my vet advised a huge insulin increase after just 5 days, that I now know would have likely been lethal.
After 142 days of insulin shots, testing, etc, Samantha went off insulin last spring and is doing great. She had an infection this summer which drove her BG up for a week or so, but once she was antibiotics it came right back into normal ranges and she is doing great.