dos right motor periodically stucked or keep dosing

[DTz]

last nite i replaced my reagents for trident and left the auto control dosing on while doing calibration. the dos went on to overdose 500 ml of calcium into my reef tank. and on n off it kept running by itself eventho i have off it on fusion. anyone experience this before? the sound is coming from the right side motor of the dos.

i never expect something like this to happen. just imagine, just need an issue like this to wipe out ur tank if ur away on holiday etc.