If the milkshake is thick and whipping around the spindle, what is the most likely cause?

The most likely cause of a thick milkshake that is whipping around the spindle is due to the feed tube being closed or having leftover sanitizer frozen into the mixture. When the feed tube is obstructed, the milkshake cannot flow freely, causing it to thicken as it gets beaten against the spindle instead of being properly blended. Additionally, if there is any leftover sanitizer that has frozen and mixed into the shake, this can also contribute to a density that prevents smooth blending. This obstruction disrupts the normal mixing process, leading to the thick and inconsistent texture described.

Other factors, while potentially contributing to a milkshake's texture, would not typically cause it to whip around the spindle in the same disruptive way. For instance, inadequate mixing time might result in an uneven blend but wouldn’t specifically cause the shake to whip; too much ice could lead to a slushy consistency but still allow for mixing; excessive milk might alter the taste but wouldn’t necessarily create the thick texture described above. Thus, the condition of the feed tube or presence of frozen sanitizer more directly explains the observed phenomenon in this scenario.