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Any thoughts of what would cause this sort of not-right-angle layer slip?
Belts not tight enough?
Random glitch in the matrix? Rostock Max v2 with all metal hot-end, printing PETG.

Assuming its a new build, I would check that the setscrews on the drive pulleys are tight.

This is called layer shifting . It can be caused by mechanical/binding issues or one of the stepper motors missing a "step" or 2. Even the nozzle snagging on a lump of plastic will cause this.

I do not like layer shifting. I do not like it Sam I Am.
I'll check the belt screws. I also think it was a bit related to a leak I had in the hotend - plastic was oozing (very slowly) out the side of the nozzle, and would form a "booger" that you can see as a little bump in the picture. Maybe if the print head hit the booger it would bind enough to miss a step.

Someone else said it could be related to print speed - that if I try to print too fast it is more likely to miss steps. What is the max safe speed for a Rostock Max v2? I've heard 30, 50, and 90 so far...

Speed wise, do it logically. Start at 30, print your part, if good go to 40, if good go to 50. You will learn the feel of your machine much better.

Doubtful that you are missing steps with stock steppers, RAMBo and a reasonable build (i.e. the carriages can move relatively freely). You should not be able to print so fast that you get layer shifting from skipped steps!

You have some form of slippage somewhere. The stepper pulleys are the first place to look. Make sure your belts are tensioned properly. Also make sure that all 3 carriages can travel freely the full length of the tower. Do this with the stepper not energized and move them SLOWLY. They actually generate current (a motor in reverse is a generator) and you do not want to blow your RAMBo. Going slow also helps you isolate the problem.

If your Onyx is in the default position it looks like the slippage is on the Y axis based on what I see in the photo. Another trick to use to isolate mechanical problems is to rotate the part and replace it. 90°rotation can tell you a lot.

Also, read the docs linked to in my signature, they'll help you get on the right track and produce excellent prints. No short cuts!

benjaminhill wrote:I also think it was a bit related to a leak I had in the hotend - plastic was oozing (very slowly) out the side of the nozzle, and would form a "booger" that you can see as a little bump in the picture. Maybe if the print head hit the booger it would bind enough to miss a step.

This is doubtful. On a delta printer if you snag something hard enough to cause the stepper/s to skip, you completely throw off the kinematics and the print head will no longer travel in the X-Y plane. It will travel in an arc and cause all sorts of havoc. In general, a hot nozzle will plow through a bugger or there will be enough flex in the arms, etc that it will plow through without skipping steps.

It also requires MANY skipped steps to get a layer shift that large. If that happened, you'd likely hear and see where the problem is.

benjaminhill wrote:Someone else said it could be related to print speed - that if I try to print too fast it is more likely to miss steps. What is the max safe speed for a Rostock Max v2? I've heard 30, 50, and 90 so far...

"Safe" is subjective. You can print as fast as you want. The side effect will be lower print quality as you go faster. It is highly unlikely that you can go fast enough with the default firmware configuration to cause skipping.