I'm using the SCL(1) & SDA(1), which are the default.
I tested it out with a 16x2 LCD which has a I2C to parallel converter.
This is what it looks like when connected to the I2C converter; the SCK line goes low, but SDA remains high.
If I leave SDA floating, I get this:
Notice that the SDA line remains low, but actually goes high during data transmission for some reason: how is this happening, since I2C is supposed to be open drain?!?