There are subtle differences. Its definitely not wrong. IMHO, its a matter of priorities. The lock washer usually improves the behavior of the bolt from coming undone. However, there are a lot of ifs and details that need to be considered.
First of all a short discussion about the individual components (lock washer and plain washer).
1.1 plain washer
In general, the plain washer helps by distributing the loads in a greater surface (see my answer to this question).
1.2 lock washer
The lock washer has two distinctive functions:
- it digs into the surfaces that is in contact
- it behaves as spring and it can maintain a minimum clamping force over a greater distance.
Especially the second one is very important (you should look into joint stiffness, because its hard to explain here), however the idea is that the lock washer behaves as a soft spring (relatively to the bolt). When you tighten a lock washer you can displace the lock washer for at least mm without significant increase in the force. If on the other hand you didn't use a washer, then the force for tightening would increase very fast upon contact of the bolt with the clamped material. The benefit is that if there is displacement induced vibration, there is almost always a residual force, that doesn't allow the bolt to rotate.
Additionally there is a difference whether its a bolt-nut or just a fastener.
Figure: blind fastener configuration
My preferred way when there is a blind fastener (sometimes called cap screw) clamping two pieces, is just using a lock washer. Only using a lock washer and tightening it sufficiently will probably create some sort of marking on the surface, which will inhibit further the untightening of the screw.
Using the plain washer, is helpful because it spreads the loads, however, if you are worried about dynamic loads, and untightening due to vibrations, the plain washer will probably won't help
2.2 bolt nut
When using a bolt nut, - in general- the preferred way (at least the one that I was told by experts), is that its best to have the plain washer at the bolt head, and the lock washer on the side of the nut.
By digging in the surface, the lock washer reduces the probability of the bolt rotating.
If the main worry is to
- secure the bolt, then just the lock washer is better (although there are better ways - castellated nuts and lock pins).
- to protect the clamped material (especially if the clamped material is soft see PLA), then I would prefer using the lock washer with the plain washer (clamped material - plain washer - lock washer - bolt/screw)
I guess the bottom line is that you need to consider the priorities on each application.