Comets will sometimes stack regolith more densely than expected. From what I can tell, this happens when the regolith would be placed in an occupied spot, so instead it is moved to a different one, this time without respecting the limits.
Comets sometimes deposit regolith through bunker tiles (as seen in screenshot). This "forced" regolith is less likely to respect limits.
Steps to Reproduce
Depends on comet luck - seems more likely when comets strike diagonally
This issue has not been confirmed by a developer yet.