Content-modification system with advertisement reconciliation feature

In one aspect, a method includes while a first content-presentation device is performing a content-replacement operation in which the first content-presentation device outputs a replacement advertisement segment in place of an advertisement segment, determining, by a computing system, that the advertisement segment is not an expected modifiable advertisement segment. The method also includes in response to determining that the advertisement segment is not the expected modifiable advertisement segment, determining, by the computing system, that the advertisement segment is ineligible for replacement. The method also includes in response to determining that the advertisement segment is ineligible for replacement, causing, by the computing system, at least one content-presentation device to perform, at a subsequent content-replacement opportunity, a corrective content-replacement operation in which the at least one content-presentation device outputs the advertisement segment in place of a subsequent modifiable advertisement segment that the at least one content-presentation device is scheduled to receive.