There are times that I think I've made more mistakes and failures than successes! Fortunately the wonderful polyclay community has saved me tons of both by their generous sharing of information, techniques and ideas. Naturally there are a number of things that you just have to learn from your own trial and error. The following are horrible mistakes I've made, that have ruined countless hours and $$ of supplies:
*If you tumble sand beads too long without checking, clay colors that do not go all the way through a bead will sand off!! It's possible to tumble beads that you've applied to clay to (like cane slices), but you have to be very careful.
*Fimo Gel scorches if it's baked too long. It covers your lovely just-finished and covered mosaic pieces with a creeping black scorch.
*Making sure you use products that have been tried and tested by many people before you is essential! In other words, don't use some varnish with a name you don't recognize and never use a spray coating.
Over the years I've tossed so many pieces into the trashcan because I've messed up so bad they can't be recovered. It hurts for a clay miser like me!!