Short answer. They have to warn you in advance. Section 50 of Rule 130 bars admission under the residual exception unless the proponent notifies the adverse party, sufficiently ahead of the hearing or by pre-trial, of the intent to offer the statement and its particulars, including the declarant's name and address, giving you a fair opportunity to prepare.
What the law says
A statement not specifically covered by any of the foregoing exceptions, having equivalent circumstantial guarantees of trustworthiness, is admissible if the court determines that (a) the statement is offered as evidence of a material fact; (b) the statement is more probative on the point for which it is offered than any other evidence which the proponent can procure through reasonable efforts
Rule 130, Section 50 — Residual exception. Read the full provision →
What the law says
a statement may not be admitted under this exception unless the proponent makes known to the adverse party, sufficiently in advance of the hearing, or by the pre-trial stage in the case of a trial of the main case, to provide the adverse party with a fair opportunity to prepare to meet it the proponent's intention to offer the statement and the particulars of it including the name and address of the declarant
Rule 130, Section 50 — Residual exception. Read the full provision →
What the residual exception itself requires
Section 50 lets in a statement that no other hearsay exception specifically covers, but only under conditions the court must find satisfied. A statement not specifically covered by any of the foregoing exceptions, having equivalent circumstantial guarantees of trustworthiness, is admissible if the court determines that (a) the statement is offered as evidence of a material fact; (b) the statement is more probative on the point for which it is offered than any other evidence which the proponent can procure through reasonable efforts. A third condition, tied to the purposes of the Rules and the interests of justice, applies as well.
The separate notice condition your question is about
Meeting those admissibility conditions is not enough on its own. Section 50 adds a distinct requirement that exists specifically to prevent the statement from being sprung on you: a statement may not be admitted under this exception unless the proponent makes known to the adverse party, sufficiently in advance of the hearing, or by the pre-trial stage in the case of a trial of the main case, to provide the adverse party with a fair opportunity to prepare to meet it the proponent's intention to offer the statement and the particulars of it including the name and address of the declarant.
What the notice actually has to contain
The notice is not a bare heads-up that a hearsay statement might come up. Section 50 requires the proponent to disclose the intention to offer the statement together with its particulars, expressly including the declarant's name and address. Timing matters too: the disclosure has to come sufficiently ahead of the hearing, or by the pre-trial stage in a trial of the main case, so you genuinely have a fair opportunity to prepare a response before the statement is offered.
What happens without proper notice
Section 50 frames notice as a precondition to admission, not a courtesy the proponent can skip and still rely on the statement: it may not be admitted under the residual exception unless that advance disclosure was made. If the other side tries to introduce a statement under this exception without having given you the required notice and particulars beforehand, that is a basis to object to its admission on that ground.