[kh99]

Change the second (broken) plugin to use hook parse_templates. Your use of a template in global_start causes the 'standard' templates (including headinclude) to be rendered, so by the time execution reaches showthread_start it's too late to set a template hook.

Edit: you may also have to enclose the code in that second template in if (THIS_SCRIPT == 'showthread') { /* code here */ } otherwise it will be on every page.