RTV silicone as solder resist
Don't let the acid (cure silicone) eat your board!

Not only will adhesion be a problem -- many RTV silicone (polysiloxane) formulations release acetic acid as they cure (the vinegar smell is a dead giveaway).  This released acid, in conjunction with any present moisture, corrodes copper at low (50-500ppm) concentrations on the order of a gram per square meter per day or worse, enough to eat through standard (1oz/35&micro;m) PCB copper in a few months' time.

Neutral cure formulations release non-corrosive compounds such as oximes and alcohols during the curing process -- most are ketoxime cure, but some specialist formulations are alkoxy cure or even ketone cure.  These formulations are generally considered safe for electronics use, and will say they are neutral cure or oxime cure somewhere on the product packaging.