Competitive Balance exists so that there is some symmetry between specific advantages and the Necessary Drawback. If someone can hit hard but is easily hurt, they are a Glass Cannon. If they are unstoppably strong but slow-moving, they are a Mighty Glacier. If they are incredibly fast but go down quickly when hit, they are a Fragile Speedster. A crippling overspecialization occurs when an obscenely powerful advantage makes a trade-off that limits how effective it is.
In some Real-Time Strategy games, unit types are often specialized beyond all reason. They often have only one weapon, effective against just one particular kind of enemy, and usually cannot defend themselves at all if their particular weapon is ineffective. The most prominent examples can be seen in this genre: explosives will deal low damage to infantrynote , while Anti-Aircraft cannons can't defend against ground units. This is usually a result of game balance. After all, if tanks are effective enough against other tanks and infantry, then why build anything else? Additionally, until recently, most units in games were depicted with only one weapon. A few might've had a second gun used for attacking enemy fliers, but that was about it.
Can result from a Tactical RockPaperScissors design scheme, from intensive Min-Maxing in a character build, or a case of Highly Specific Counterplay. People who do this for Elemental RockPaperScissors are suffering from Poor, Predictable Rock.
See also Anti-Air and Strong Flesh, Weak Steel. Compare to An Adventurer Is You, where this is used to force players to work together. See also Sacrificed Basic Skill for Awesome Training for a characterization equivalent and Single-Task Robot for a robot equivalent. Severely Specialized Store is a comedic variation.
Don't confuse "having a weakness" or "not being the best at everything" or even "not being quite good enough" as being this trope. Don't be tempted to list something because it failed in some way unless you can point a finger at its crippling specialty!
The opposites of this trope are the Red Mage, Jack-of-All-Stats, and Master of All. At the other extreme that loops right back is Master of None, where the character has no great specialization... but sucks at everything anyway.