Last update 31 Jan 2010. A randomish list of miscellaneous things that can cause trouble. I will add to it as I think of them.
DECT phones. When disconnecting these for connection problem diagnosis some (older?) ones need to be unplugged from the mains as well. I don’t know why but there are several reports of this having an effect. Possibly it is radio emissions from the power supplies.
Plug-in phone extension cables. There can be at least two problems with these.
First they usually contain a ring wire. See Ring/Bell wire in Troubleshooting.
Second they are often of poor quality if bought cheaply from DIY shops and can in themselves downgrade the broadband signal. You need proper telephone extension cable such as CW1308.
“New-build” houses and flats/apartments where the builder has installed the extension cabling are frequently problematical.
From the NTE (Network Termination Equipment) point then the builder can do what they like. Cheap burglar alarm-type cable is frequently used with possibly disastrous results for broadband.
The very latest NTE is external, see Useful Links, so even the normal internal main socket may be affected.
Things affecting noise margin other than in the main pages and above.
Faulty street lights; some central heating systems; dodgy microwave ovens; Christmas lights, could be yours or even a neighbour’s; nearby (electric) railway lines - and automatic level crossing equipment on any line; faulty filters - they can fail; some Sky boxes - they can need double-filtering; wrongly wired or unfiltered phone-connected burglar alarms.