Updated: Apr 5, 2020
Chocolate is a pretty particular entity, and it's just not very pleasant when it gets out of whack---it can be grainy, dull, bendy, splotchy, streaky, thick, globby, or sticky and melty. But there are only two main causes for chocolate to get messed up while you're tempering it and we have three ways each you can fix it. The first problem is un-tempered through overheating and the second problem is over-crystallized (too thick to work with but in temper).
Un-tempered due to Overheating
For dark chocolate that means your chocolate is higher than 94 degrees and therefore not in temper. Maybe you can see fat streaks, or that the emulsions isn't smooth. Maybe it's just too liquidy, and you're getting a big foot on your enrobed or dipped pieces. Here are a couple ways to fix it.
1) Add more seed (small pieces of tempered chocolate).
2) Keep stirring. The agitation and time that passes can encourage your cocoa to crystallize correctly. (Could also be achieved by tabling a small portion).
3) Start over - reheat your chocolate all the way to the melting point (typically 122 for dark chocolate). And continue to temper from there, using the seed method.
Thick, slow moving chocolate that sets up too fast. May coat your products unevenly.
1) Warm it up - use the oven, or a heat gun, or a microwave (but be careful!)
2) Add some melted chocolate. Make sure your melted chocolate is at its melting temperature, meaning for dark chocolate, it needs to be at it's melting temperature of 122 degrees Fahrenheit. That ensures that there are no unstable cocoa butter crystals already formed that could taint your tempered chocolate.
3) You may need to start over, but first consider increasing the working temperature by 0.5 - 1 degree Fahrenheit higher. For instance for dark chocolate, go from 90 degrees to 90.5 or 91. This increases your risk of untempering, so continue to check your other identifiers throughout (namely the viscosity and set tests).
Elevate your bowl of chocolate so that it's off a cold surface, that way you have more time while it stays at the working temperature. (Easy way is with a "fluffy" ring of tinfoil).
Work in as large of a batch as you can, so that you have the longest time available that your chocolate stays in temper.
Even if your chocolate appears to be unusable, you may be able to strain out lumps and use the chocolate in cake or incorporate it into a buttercream. Chocolate is expensive so unless it's burnt, it can probably be saved for something!