Dough that hasn't fermented long enough -- it'll be dense, potentially gummy, and may have large irregular holes near the crust (fool's crumb).
Underproofing is the most common beginner mistake. Signs in the finished bread: dense, gummy crumb that feels heavy; large air pockets near the crust with tight crumb in the center; a strong yeasty smell; pale crust. The fix is simple -- ferment longer next time. Use an aliquot jar to track your dough's rise, and push the fermentation a bit further than you think you need. Most home bakers underferment rather than overferment.
