In South Africa, limited studies have been conducted investigating responsive teaching and little is known about how teachers respond to unexpected events 'in the moment' that did not form part of their planning. In this article, we report how a Grade 9 novice and expert teacher responded to unexpected learner offers during the teaching of algebra using a qualitative case study approach. Three consecutive lessons for each teacher were video recorded, transcribed and analysed. Our units of analysis for episodes were teachers' responses to unexpected learner offers and we coded the responses as 'appropriate' or 'inappropriate'. Indicators used to highlight the degree of quality of the response were 'minimum', 'middle' and 'maximum' if a response was coded as appropriate to a learner's offer. Once lessons were analysed, the first author conducted video-stimulated recall interviews with each participant to gain insight into the two teachers' thoughts and decision-making when responding to unexpected learner offers. The findings from this study illustrated that the novice teacher failed to press learners when their thinking was unclear, chose to ignore or provided an incorrect answer when faced with an unexpected learner offer. Conversely, the expert teacher continuously interrogated learner offers by pressing if a learner offer was unclear or if she wanted learners to explain their thinking. This suggests that the expert teacher's responses were highly supportive of emergent mathematics learning in the collective classroom space.