A system comprising a catheter and a processor, said catheter comprising: a shaft for insertion into a heart of a patient; an expandable distal-end assembly, which is coupled to the shaft and is configured to make contact with tissue of the heart; at least first and second electrocardiogram, ECG, electrodes, which are coupled to an outer surface of the expandable distal-end assembly, and, when placed in contact with the tissue, are configured to sense ECG signals in the tissue; and a reference electrode, which is positioned within an inner volume of the distal-end assembly, and wherein in an expanded position of the distal-end assembly, the reference electrode: i) has no physical contact with the tissue, and ii) is positioned at a first distance from the first ECG electrode and at a second distance from the second ECG electrode, wherein a difference between the first and second distances is smaller than a predefined threshold, characterised in that the processor is configured: to calculate, based on the ECG signals sensed at one or more positions on the tissue, one or more respective local activation times (LATs), which are indicative of an electrophysiological wave propagating in the tissue; and based on the calculated LATs, to display to a user at least an annotation indicative of a timing error in a rhythm of the heart. a shaft for insertion into a heart of a patient; an expandable distal-end assembly, which is coupled to the shaft and is configured to make contact with tissue of the heart; at least first and second electrocardiogram, ECG, electrodes, which are coupled to an outer surface of the expandable distal-end assembly, and, when placed in contact with the tissue, are configured to sense ECG signals in the tissue; and a reference electrode, which is positioned within an inner volume of the distal-end assembly, and wherein in an expanded position of the distal-end assembly, the reference electrode: i) has no physical contact with the tissue, and ii) is positioned at a first distance from the first ECG electrode and at a second distance from the second ECG electrode, wherein a difference between the first and second distances is smaller than a predefined threshold, characterised in that the processor is configured: to calculate, based on the ECG signals sensed at one or more positions on the tissue, one or more respective local activation times (LATs), which are indicative of an electrophysiological wave propagating in the tissue; and based on the calculated LATs, to display to a user at least an annotation indicative of a timing error in a rhythm of the heart. i) has no physical contact with the tissue, and ii) is positioned at a first distance from the first ECG electrode and at a second distance from the second ECG electrode, wherein a difference between the first and second distances is smaller than a predefined threshold, characterised in that the processor is configured: to calculate, based on the ECG signals sensed at one or more positions on the tissue, one or more respective local activation times (LATs), which are indicative of an electrophysiological wave propagating in the tissue; and based on the calculated LATs, to display to a user at least an annotation indicative of a timing error in a rhythm of the heart.