それは彼ら側での間違いだった。

that (indicating an item or person near the listener, the action of the listener, or something on their mind); it; that time; then; there (indicating a place near the listener); you

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

indicates location of action; at; in; indicates time of action; indicates means of action; cause of effect; by; and then; so; indicates continuing action; indicates certainty, emphasis, etc.

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

mistake; error; blunder; accident; mishap; trouble; improper conduct (e.g. between man and woman); indiscretion

be; is; indicates past or completed action; indicates light imperative