Don't pull a washing machine out before you check that there is enough slack in the hoses, else you risk snapping the hoses and causing a flood;
Don't bypass any safety devices like fuses, door locks or overload protection (TOC) etc.
Don't turn a washing machine upside down. This can cause water to run onto electrical components and risk electrical faults or even endanger you. Always be careful to ensure a machine is fully drained before you lay it down - and it's probably best to do so on the machine's front or back, not turn it upside down;
Don't leave your machine running unattended after attempting a repair because it may malfunction or develop a new, more serious issue that presents a danger to you or your property. Always test it carefully and thoroughly for leaks or faults after a repair;
Don't leave the cover off a washing machine when it's connected to the mains power. A pet or a person could easily gain access to lethal current.
Don't attempt work on a machine that is currently connected to the mains and make sure that there is no residual current in the machine. Note that same components can still give you a shock even after the machine is disconnected from the power. Always double (or triple) check that it is safely isolated from the power and do it yourself (don't trust someone else telling you they already disconnected it).