X10 Powerline Network is Not Like Ethernet: X10 is old technology and it is quite slow by modern standards. Commands will normally take at least 1 second apiece to complete, and until one command is completed the next one can not start. Multiple controllers can co-exist on the same powerline, but if more than one controller tries to talk at once there will be a collision; in most cases neither controller's command will succeed. There is no retry provision in the protocol, so when collisions occur things just fail to happen as expected. For this reason it is preferred (although not required) that the CM11 / CM12 connected to the computer running X10 Plugin be the only controller in the house. Repeaters and other X10 "boosters" can also, at times, cause collisions, particularly when a long series of X10 commands is being sent as quickly as the CM11 / CM12 can process them.
Macros Can Cause Problems: It is possible, by using the X10 Active Home tool, to load "Fast Macros" into the CM11 / CM12 EEPROM memory. The macros run when a triggering event takes place (such as a time of day). Macros are useful and are not actually incompatible with the X10 Plugin software, but they can cause intermittent problems. Put simply, the CM11 / CM12 can only do one thing at a time, and it gives macros a higher priority than commands from the computer. So if a macro trigger happens to fire at the same instant the computer starts to send a command, the macro will run and the computer's command will get ignored. This is not a huge problem but is can be a bit difficult to diagnose. Temporarily clearing all macros out of interface memory is a good troubleshooting tactic if you are having occasional failed commands.
CM11 / CM12 Interface is Not Infallible: Under certain unusual circumstances, CM11 / CM12 interfaces can and will lock up. Most commonly, lock-ups happen when another controller (e.g. a keypad) begins to send on the powerline at the same moment the computer begins to send to the interface. Once the interface is locked up, nothing sent from the computer will work. To unlock it, stop the X10 Plugin and then send a few commands on the powerline from another controller (e.g. push a few buttons on your keypad controller). Then start the X10 Plugin back up. This technique almost always works. Note that this sort of lock-up can't happen if the computer interface is the only controller on the powerline.
Dimming is Not Intuitive: X10 Lamp and Wall Switch modules can not be ramped up gradually from off. The hardware insists upon coming on at full brightness when a turned-off module receives a Bright or a Dim. Bright behaves just like On: the light comes on at full power and stays there. Dim turns the light on at full power first and then ramps it down by the percentage you asked for. A light that is dimmed down to zero brighness is still considered to be on. Sending that light's module an On command does nothing; you must either turn it Off and then On, or use a Bright command to ramp it up to some brightness. Also, as mentioned earlier, dimming is relative (Dim-BY) not absolute (Dim-TO), so there is no way to directly set brigtness to a predetermined level.
All of the Above are restrictions imposed by the X10 hardware and protocols, not limitations in the X10 Plugin software. Experiment with your X10 gear using a hardware controller (e.g. a keypad) and see for yourself!
Have Fun with this Plugin and your X10 systems. But keep in mind that X10 is not 100% perfect. So please don't try using it to control your stove, your electric saw, the launching of ballistic missiles, or anything else that could start a fire or hurt somebody if it operated unexpectedly due to a glitch or the actions of a visitor on your Web site!