My main peeves are unnecessarily large sub-peak cairns and any painted nonsense.
Couple of times in clag I've assumed I was at the top thanks to a huge cairn with only some careful map checking preventing a missed summit (Loony Bin springs to mind).
Any painted stones etc. get flung in the nearest remote corrie (found one commemorating a 75th Munro, I mean come on!).
Might also remove a jauntily perched stone on top of a cairn or a few off a too-big-for-the-hill cairn.
On reflection, I may just be a grouch....
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