Take your ohm meter and measure the filament resistance at room temperature, with no power applied. Now, assuming you are using a standard incandescent light bulb (say somewhere around 50 to 100 watts), look what happens to the power consumed at the moment the bulb is first powered up and the power that is consumed after the filament heats up. P=E*E/R The wattage rating of a light bulb is measured at full rated voltage in steady state conditions. That means a 100 W bulb pulls quite a bit more than 100W at the instant it's turned on. You know how it seems that an incandescent light bulb always burns out when you flip the switch to turn it on.
So, the result is that as you are keying your light bulb filament (dummy load), the actual load seen by your transmitter rapidly varies. It starts low and goes to a higher value with each dit and dah.
The old tube rigs didn't much care – they were tough. But your output transistors are a bit more finicky in their diet. Oops! There goes that magic smoke again.
But for QRPer's - not to worry too much. At 5 watts (figure your voltage into 50 ohms), a 50 to 100 watt light bulb probably won't undergo too much heating. If you have enough power to make the filament glow (even dimly), that load resistance is already on the way up.
What it might do, though is contribute to a “yooupy” signal, if your rig is sensitive to a changing load. Remember how the changing load presented by a blowing wind on a wire antenna could affect the old single tube MOPA transmitters and regin receivers back in the days of yore?
Oh, well... Enjoy! That's what ham radio is all about, anyway.
Bruce – KK0S