Most of the time, 100 ohm diff will still work. But, when it doesn't work, it will be intermittent and cause blood to shoot out of the ears of your software nerds. 100 diff pair advantages don't exist without a differential transmitter and receiver on both ends. Without that, you are just coupling noise from your clock line to your data line. May not notice at short lengths. But, after a certain length, you will have random intermittent things happening. You don't want that in production. Usually what causes me to grip the desk tightly is engineers running I2C through multiple boards and none of the 100-mil headers have ground pins adjacent to these edge-sensitive lines. I know software is going to blame me for that.
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