This a scenario where a player presses declaration beforehand, the cards are not arranged in logical sequences (Pure sequence, impure sequence, sets, etc.). Here the player ends up losing completely and his opponent wins hands down.
Examples of common invalid declarations:
Incorrect declaration with Invalid Sequences
Example 1: 3♠ 3♥ 3♦ 3♣ | 2♠ 2♥ 2♦ | 7♠ 7♥ 7♣ | 8♥ 8♦ Joker
Although this set has cards of the same value with different suits, this is considered invalid simply because a valid declaration requires 2 sequences, a pure sequence (without a joker) and an impure sequence followed by one or two sets.
Example 2: 7♥ 7♠ 7♦ | 2♥ 3♥ Joker | 10♠ J♠ Q♠ Joker | 2♠ 2♥ 2♦
In this example there are two Impure sequences and one set, therefore it is not a valid declaration. To make a valid declaration you must have at least one pure sequence.
Example 3: K♥ K♠ K♦ | 2♥ 3♥ 4♥ 5♥ | A♠ A♥ A♦ | 3♠ 3♥ 3♦
In the third example, there is one pure sequence and three sets, you cannot make a valid declaration because as per rummy rules there should be one Pure Sequence and one Impure/valid sequence.
Incorrect Declaration with Invalid Sets
Example 1: 5♠ 5♠ 5♦ 5♣ Q♥
In the above example, the first two 5s are of the same suit even with the wild card joker this set is considered invalid as it has two cards of the same suit.
Example 2: Q♥ Q♥ Q♦
The is also an invalid set as the first two Qs are of the same suit.