I/O question

A 3.3V input buffer with 0.5V driving it will not cause any problems. If setup as a TTL compatible input, it has to accept anything lower than 0.8 V as a valid logic low input.

Where things get interesting is in the no-mans land between 0.8V and 2.0 V. Here both the P and N transistors are on somewhat, and you will get switch-through current. If the transistors are sized properly, this should never damage the input buffer, but it will cause higher current draw, and may cause oscillations, metastability and other issues for logic on the other side of the input buffer.