I've found what I believe is the serial number for a possible 1990 Cannondale SR800.

The serial number appears to say 86009880044. I looked at the serial decoder on VintageCannodale website but it is supposed to start with the frame size and I don't think 86 can be right?

From 86 to 92 Cannondale used two different serial systems. Your frame appears to have used the second system:
Single digit spacing (0= 120 or 130, 5= 135, 6= 126)
Double digit frame size in mm for racing frames, inches for touring frames
Four digit date code
Four digit sequence number

My guess is that you have either misread the first digit or it is incorrectly stamped. Measure the rear dropout spacing. The rest then follows:
60 cm frame
Built 09/88
44th frame built that day.

Which makes it an 88 or possibly very early 89 model year frame.

Thank you for the response! Interesting, the 8 is very clearly printed there. How common is it that they incorrectly stamped it?

I can't post proper links here because I haven't posted enough, if you just take the spaces out this a link to a pic of the serial number: h t t p s://imgur.com/oHb7LaG

Well, that’s clearly an 8 in the first digit position. Beats me.
What’s the spacing? In 1988, Cannondale was starting to use 128mm spacing on some frames to accommodate either 126 or 130 hubs.
If this is a 128, could be the person stamping the numbers thought that was the correct code.
Another thought: I’ve never seen the ‘later’ serial number system used on an 88 era frameset.
Remove the 8, and it correctly reads like the earlier serial system, assuming it’s a 60 cm racing/criterium style frameset.
When you get enough posts, put up pics of the entire bike.