I only found it recently, but I wonder if anyone can spot (may need the larger image) the slight 'frame building' mistake on my Saracen...
Nothing serious, but a bit irritating.
A couple of idiosyncrasies might qualify.
There was a fashion for putting the seatpost binder bolt in front of the seat tube. Apparently to minimise water/ dirt getting into the seat tube but there was a greater incidence of fatigue cracking as a result. My fat thighs tend to annoyingly rub against such QRs.
Extending the top tube past the seat tube is just a styling exercise but it worked for GT.
Putting the rear gear cable along the top of the seat stay guaranteed it would collect water and muck but was much more common than routing it underneath the seat stay.
The seat tube bidon bolts should be a shade lower to avoid clashing with the front mech clamp, particularly if the biggest chainring size alters.