The controller and I vetoed several candidate control sites because of the 60-metre rule, and we also made adjustments because of the 30-metre rule. But neither of us thought it appropriate to object to this pair of features. Perhaps we need to be more cautious in future.
Note that BOF Planning Appendix B advises "Be pessimistic when interpreting 'features which appear similar in the terrain'" and also states (#6.2.6)
Other combinations to avoid include ... depression/pit/shallow re-entrant
So, even a shallow re-entrant and a pit are to be avoided if they are <60m apart! ISOM pits might have steep walls in theory but I'm sure we have all seen smooth sided and shallow hollows mapped as pits in practice ...
The guiding principle should be that if there is the slightest chance that feature A might appear similar to feature B in the terrain, then they have to be at least 60m apart.