For my nightmare it wasn't a bike but a car ....namely a seat ibiza cupra 1.4tsi jobby.
Bought it brand spanking new on the first day of ownership it developed a rather annoying gearbox fault in that if you pulled up to a junction or set of lights and selected neutral (8 speed dsg gearbox) and tried to select drive it would shit itself requiring a switch off and back on to get it working again.
Straight back to the garage for a software update..... which did actually sort it.
Roll on a couple of weeks in to ownership I notice it was using oil (checked on the dipstick from cold and always in same location) so I took it back to the garage to be told nah mate can't see any issues and that it had only consumed around half a litre but when told I'd only done around 100 miles that week to and from work it still didn't register as an issue with them)
So I take back the car to use but I kept a very keen eye on the dipstick before each drive to measure when it dropped and the worst day was after a 7 mile drive to work check it at home before leaving it was at the top mark on dipstick when I came to leave work I check again to find it didn't even register on the dipstick so off back to the garage on the way home for them to top up with 1½ litres, and still they wasn't fazed by the 7 miles I'd driven between checks even though I said there's no way that is within spec for oil consumption but they took it with a pinch of salt so away I drove again after they topped it up.
It continued in the same vein week after week so God knows how much oil it actually consumed but I'd say 100s of litres in the 18 months I owned it.
At the 9 month mark I took it in with oil literally dripping out of the tail pipe so I asked them if it was supposed to be doing that and got told it was just soot ......I didn't know soot dripped
I had well and truly had enough by this point so I gave them an ultimatum that they either change the engine or I'd drive it through the front of the showroom so it had a new engine and turbo.
Prior to this I was told it was my driving I was racing it from cold but with video footage they then told me im not driving it hard enough......wtf
The final straw was on our Cornwall holiday journey when in torrential rain on the m4 the wipers just cut out completely needing half a dozen fuse changes to complete the journey. After that I got the car recovered from Cornwall and replaced with a hire car and told them I was walking away from the vehicle and finance.
It's put me right off owning anything new vehicle wise