This videos explains what could go wrong using a schlenk line.
It should be avoided to:
Leave flasks unsecured. If it drops, the mixture can get into the tube und get contaminated.
Open the vacuum too quickly. If the valve is opened too quickly, an abrupt boiling could occur, thus leading to a spreading of the mixture within the flask and the tube. If a solvent shall be evaporated into the vacuum, it's important to open the valve carefully and slowly.
Underestimate interdependencies of connected parts. If the inert gas flow to an evacuated flask is opend an underpressure occurs in the whole schlenk line. This could lead to an abrupt boiling or foaming in connected flasks. If a septum is used on a gassed flask and an evacuated flask is opened, oxygen can flow through the septum into the set up, because septa aren't usable for vacuum applications. So all valves have to be closed, when gassing a previously evacuated flask.
Change the pressure during a distillation. Generally it's important never to seal a distillation apprature, but to keep it open to the inert gas line. If the pressure is reduced, e.g. by opening an evacuated flask to inert gas, the mixture can boil up the . The procedure hast to be started over again .
Reduce the pressure during heating to reflux. The mixture will boil up the cooler.
Gas a set up beneath a bottom cap or evacuate above a bottom cap. If Celite is used, it will be blown through the apparatus