Issue: Autumn in the UK means hedge clipping time – local councils and farmers clipping their hedges to keep them nice and tidy.
Damage: Rural roads and tracks strewn with thorns and jagged twigs. Expect serious punctures, broken hangers, bent derailleurs, bent spokes and general problems with things catching in things and things. At worst, being stranded in the middle of nowhere.
Remedial action: See specific pages for punctures, hangers and so on. Punctures can be easily fixed at the roadside (although for giant thorns it can be best to leave them in until you’re past the clipped section if the tyre hasn’t already deflated). Hangers can be replaced if you have a spare with you. As for larger problems, I once freewheeled off a hill with my chain in my bag and my derailleur in my pocket, and on another occasion removed a section of chain to set my bike up as a sort of singlespeed, although the chain snapped again shortly afterwards.
Ways to avoid: There’s not a particularly good way to avoid the issue. Manmade trails should be absolutely fine, but since Autumn is likely to be the last time to get in some natural riding before Winter it’s a shame to limit yourself at this point. On roads you can sometimes do your best to weave between the worst bits of debris, but on tracks this is more difficult. Punctures aren’t too much of an issue. Broken hangers are a little more serious, and if you have the ill luck to hit something that breaks your hanger, snaps your chain, and bends your derailleur then you’re basically looking at a walk home.