This is some info I wished I had found earlier in the day, so hope this helps someone else...

while fiddling with a contribution that uses a jscript_...php file to load two .js files I thought why not just rename the two.js files to jscript_whatever.js and ZC will load them automatically: a whole file saved! How clever that would be!

So I renamed

instant_search.js ->jscript_instant_search.js
jquery.js->jscript_jquery.js

and it would not work.

Although both files got loaded ok, the first function call to jquery failed.

No doubt some of you are chuckling, for the rest of you..

i comes before j

so the scripts were loading in aphabetical order and the function was executing before the jquery was loaded.

Renaming to
jscript_instant_search.js
jscript_0jquery.js
fixed it.