I accidentally mixed up the Loop Effects Send & the second Speaker Output of my Amplifier. Is it serious?
Yes it is. Always be careful not send the speaker signal into a pedal or an effects processor. Even as both are outputs providing a signal, there is a big difference.
The output of an effects loop (the "send" or "pre-amp out" depending amplifiers) is about a few millivolts. To get a bit more technical, the norm is 750 mV equal to 0 dBU. In contrast, the speaker output of an amplifier is about ten volts ...
We can calculate the power of a speaker output but you have to know that for a 100 watts amplifier, you may have until 20 or 30 volts or more at the output.
Imagine now, 30 volts instead of 0.750 volts. Forty times more. Imagine what can happen to the input of a stomp box or even on your 19" rack effects processor - the input circuit of the effect device will quite simply burn out.
Please always be careful anytime you plug in your effects in the effects loop of your amplifier, and be sure to double-check your which socket you're plugging your cable into.