It is a well known fact that Ammeter is always connected in series with the element through which current is to be measured. Figure below shows two cases, in one case ammeter connected in series while in other case, ammeter is connected in parallel.
As an Ammeter has very low internal resistance therefore ammeter connected in series so that all the current flowing in the circuit element must flow through the ammeter in order for it to measure the current. So if we connect it across i.e. in parallel to any circuit element, we will be shorting that element. Because of this shorting of the element, the current through the circuit will increase which will definitely burn the meter. If MCB is installed in the supply, then MCB will trip due to over-current. The above scenario is depicted in figure above.
Now, we know that voltmeter is always connected in parallel to the element across which potential difference i.e. voltage is to be measured. This is done so as our aim is to not to alter the value of current flowing though the circuit rather to measure voltage drop across the element. An ideal voltmeter has infinite resistance.
Well as discussed, a voltmeter has a very large internal resistance. It is connected in parallel to the circuit element so that all the voltage is dropped across it and it can measure the voltage. So by connecting it in series, we will be reducing the current to nearly zero. On other words it will lead to an open circuit.
Therefore care must be taken while measuring voltage using multimeter. It must be ensured that multimeter is selected in voltage mode before measuring voltage between two points. If by mistake, multimeter is selected in current mode and we measure the voltage between two points, this simply means we are connecting ammeter in parallel which means high current through the meter as well as circuit. Modern Multimeter has inbuilt fuse, which will burn in this case. But for personal safety, it must be ensured that proper selection in Multimeter is done before the measurement.