In reply to Chris the Tall:
I've a lot of experience in this neck of the woods. Whilst it is a trail centre, it's also a big multiuser venue with lots of cycles, motorbikes pedestrians and horses in all parts of Clipstone. There have been incidents/problems/ tensions with MTBs and pedestrians in the past, and periodically there are meetings and campaigns to try and smooth things over.
Caltrops are a real nightmare because they are hard to find, so even if you know they've been laid in an area, getting rid of them can take forever, plus there are all sorts of liability issues. This is by no means uncommon in Notts, they've been laid here before, and also at places like Attenborough nature reserve and the old railway trails around Mansfield.
In small consolation, most of the ones historically have been laid for motorbikes, but they don't discriminate. There have been neck height wires, hidden obstructions and other vandalism as well in the past, particularly when Eakring Lane and the "desert" were seeing more traffic.