The situation is that you have a repetitive bassline to-do, which involves some movement but in short loops (so not dacota or something), say a ska bassline or, quite often (in fact almost always) I get this in Billie Jean. In Billie Jeans case, your left* hands job is just to play the same 8 note run on just 4 notes, Gb Db E Gb E Db B Db, over an over again, until you get to the chorus for a little break, or the singer remembers its the end of the song. So you tell your hand to play that, and it goes off and does it.
And then, the 357th time it has to do it, it decides to do something totally different for no readily apparent reason, or just completely forgets how to do it and completely throws you, because you are not really thinking about it anyway, by then you have normal gigging thoughts like 'you better not spill that pint on my pedalboard', 'did your parents really let you come out dressed like that?' and 'yes, your singing voice is lovely, why don't you start your own band'.
I suppose that is why it happens, but it can be really offputting. Does anyone else get this?
* other hands are available if you are built backwards