• Solvent evaporated during refluxing due to a poor seal.
• Stirring stopped.
• Reaction became heterogeneous during slow addition, complicated mixing.
• Slow addition mechanism stopped, so reagent was not added to the flask properly.
• The desired reaction occurred cleanly, but then (a) a byproduct started to form, so the reaction appears to be nonselective (b) the product decomposed under the reaction conditions.
• Unwanted fluctuation of temperature
• You forgot to add a reagent or miscalculated, and no reaction occured at all. You could have corrected the problem, but you didn't notice in time.
• You don't know how long the reaction took.
• You don't know if the reagent was consumed at the same rate as the substrate, or if it just gradually decomposed on its own.
• You will not see the formation of intermediates, which can be an important indication of reaction progress.