Reversing carbocation and carbanion stability: The most common GOC error. Carbanion stability is the EXACT OPPOSITE of carbocation stability. $CH_{3}^{-}$ is the most stable carbanion; ($CH_{3}$)_{3}$C^{-}$ is the least stable.

Misidentifying hyperconjugation as involving C-C bonds: Hyperconjugation specifically requires C-H sigma bonds alpha to the cationic center or π bond. C-C sigma bonds contribute to the inductive effect, not hyperconjugation.

Forgetting the second condition for geometrical isomerism: Both conditions must hold simultaneously — restricted rotation (double bond or ring) AND different groups on EACH doubly-bonded carbon. $CH_{2}$=$CH_{2}$ (C=C) has no geometrical isomerism because both H atoms on each carbon are identical.

Confusing +M and -I effects of -OH: -OH withdraws electrons through sigma bonds (-I effect) because oxygen is electronegative, but donates its lone pair through the π system (+M effect). In benzene, +M dominates, making -OH ortho/para-directing.

Using wrong locants in IUPAC naming: Always number the chain from the end nearer to the principal functional group, not the end nearer to any substituent. The principal group gets the lowest possible locant.

Mistaking functional group isomers for the same compound: Ethanol (CCO) and dimethyl ether (COC) are both $C_{2}H_{6}O$ but are entirely different compounds with different chemical properties. Always write structural formulas to compare.

Assuming free radical stability mirrors carbanion stability: It does not. Free radical stability follows the carbocation order: 3° > 2° > 1° > $CH_{3}$·, stabilized by hyperconjugation and +I effect.

Mixing up E and Z descriptors: Z = zusammen (German: together) means higher-priority groups are on the SAME side. E = entgegen (opposite). Always apply CIP rules to assign priority before determining E/Z.

Ignoring s-character effects on bond properties: Higher s-character (sp > sp2 > sp3) means shorter, stronger bonds and more acidic C-H. Alkynyl C-H (sp, pKa ~25) is far more acidic than alkyl C-H (sp3, pKa ~50).