Just like the title says, and it's really easy to reproduce, especially in landscape mode. Pick two keys on the keyboard, for example, S and K. Alternate pressing them very quickly with both thumbs, e.g. sksksksksksksksk. Sometimes one or more of the keys won't register, although I can clearly see them lighting up when I touch them. Also, in this example (using S and K), sometimes I see the "G" key light up for no reason -- my fingers are nowhere near it! 
Using a Nexus One with the stock 2.1 firmware.
Using a Nexus One with the stock 2.1 firmware.