I've heard that leaving a pin floating on an MCU when configured as an input (vs. the default output) is bad for the pin, and can eventually cause it to fail prematurely. Is this true? N.B. in my instance the pin is floating somewhere between 0.3V and 1.3V due to an incoming video signal. This sometimes falls in the no man's zone of 0.8V - 2.0V when operating from 3.3V.
Leaving a pin configured as an input floating is dangerous simply because you cannot be sure of the state of the pin. Like you mentioned, because of your circuit, your pin was sometimes LOW or sometimes in no-man's land or could sometimes go to HIGH.
Essentially, the floating input WILL definitely cause erratic chip operation or unpredictable behaviour. I have noticed some chips froze by simply moving my hand closer to the board (I wasn't wearing a ESD wrist band) or some would have different startup behaviour each time the board would powerup.
This happens simply because if there is external noise on that pin, the pin would oscillate, which would drain power as CMOS logic gates drain power when they switch states.
Most micros nowdays have internal pullups as well, so that could prevent this behaviour from occuring. Another option would be to configure the pin as an output so it does not affect the internals.
It's a little worse than just being in an unknown state, or toggling needlessly. Digital circuits nowadays are mostly of CMOS type, with transistors switching both high and low sides; when we have clear 1s and 0s, they are either off or saturated, the two most efficient states for the transistors to be in. In between, however, is a region of linear operation; it's used for analog amplifiers, but it is not as efficient as the extremes - meaning more power is wasted as heat in the transistor. In the worst case, both the high and low side transistors leak thus (because the pin is in fact neither high nor low), and they can then combine to cause a notable current within the chip as they try to drive the internal state both high and low - possibly doing the same to the next gate in a chain reaction. The heat could become a problem even if the power isn't. IntelliChick's solutions still apply.
For pins also connected to ADCs, some microcontrollers offer the function to disable the digital input buffer, to prevent both this problem and leakage distorting the signal.
In practice the main effect is increased power consumption. If a pin is actually floating as opposed to connected to some indeterminate voltage source, it is possible for oscillation to occur, which as well as increasing power draw may introduce noise into other parts of the system. Any pin which has the ability to be used for an ADC or comparator input will have the facility to disconnect the digital input buffer to avoid this problem. (DIDR on AVR, ADCON1/ANSEL on PIC)
Generally it is a bad idea to leave input pin as floating as this may cause:
a) Functional problems - unknown input state, toggling (for example may trigger interrupt with undefined ISR that would hang processor)
b) Increased power consumption - most likely the input gate is similar to CMOS inverter. With this structure when the input is far enough from either rail (for example at half supply) significant cross over current will flow constantly.
c) If the cross-over current will flow the phenomena know as hot carrier injection may actually decrease lifetime of the device. The input gate may be designed just for normal switching not continuous conduction so device may fail catastrophically. Note however one would need place device in such condition for many hundreds of hours at elevated temperature for this to happen.
Note the a) and b) are real problems that one most likely one will encounter. As for c) it is less likely problem to happen but why take risk?
Some high-speed CMOS devices may be destroyed if an input is left floating, but the most common problem one will observe is increased current consumption. On PIC series microcontrollers, the extra current is on the order of hundreds of microamps per floating pin. Not enough to cause device damage, but enough to severely impact battery life in an application that would otherwise draw 5uA. Some chips have options to disable a digital input; if an input is disabled, it may freely be left floating.