Different level TTL driving each other? A problem?
If you have a 5-V device with GPIO configured as "pull-up", you will likely have this pull-up as equivalent to 30 - 70 k&Omega; resistor.

When you connect this "input with pull-up" to a 3.3V-powered IC, it is very likely that the current from 5-V will flow into the 3.3 V device via protective ESD structures (ESD clamps). Assuming the clamp will take 0.7 V (standard diode), you will have 5V - 3.3V - 0.7V = 1V difference across the pull-up resistor. 



Which doesn't make much of a current, 1/30k = 30 uA maximum. It won't hurt anything, and the 3.3V GPIO likely can withstand much more - that's where "5-V tolerant" comes.

However, the problem usually arises when this 3.3V device gets unpowered (sometimes, for power saving reasons etc.). Then the leakage from the 5-V side pull-up will charge the 3.3V IC power rail to some intermediate level - 1.5- 2V or something, which is very bad for internal POR (Power-On-Reset) circuits, and likely will result in ugly intermittent malfunction of the entire design.