Buying 64/sec -> 128/sec breaks the game ;-;
Yup, I was just about to post this one. Also buying value 6 breaks the game in interesting fashion - the coins keep dropping, but you can no longer collect them.
Width and speed though seem to simply keep charging you money at the max but not upgrade. (just in case you were thinking of going back and trying them to see if the other maximums were broken - I just finished that second run for that purpose, so I'll save you some time ;) )