you can get an 'invalid target' if your clone has not received the new target data... and still has the previous target. this is an effect of network latency. if, while on your main, you select a target and fire... in a very short time frame... your alt will receive the 'fire' instruction via keyclone within 1ms... but the new target takes at least 2 * your current latency to arrive at your clones.

i hope that makes sense.

double tapping the fire key would generally be enough to insure the clones get a target and fire. (time between taps being greater then 2*latency, of course)