OK, I'm a glutton for punishment.
Quite some time ago I purchased some small chips designed to drive a couple of LEDs at a constant (programmable) current. I was silly enough not to take enough notice of the package to order them in a BGA package.
This has spurred me on to try to improve my toner transfer technique to produce boards with traces of 0.1mm in width. For the imperial amongst us, that's a shade under 4 thousandths of an inch.
My existing printer 300dpi could not get near...