Thanks for your help, if anyone knows of a bug that might cause this please let us know.
I usually use Visual Studio (which has Microsoft project files, not Qt .pro files), but on the few occasions when I use Qt Creator I have observed that you do not need to run qmake unless you've added new GUI files to a project and the Makefiles need to be updated. Making changes to a .ui file doesn't change the set of files in the project, and the build rules -should- take care of it.

On the other hand, if you do experience unexplained build behavior, re-running qmake is often a good solution. I don't know if it is a bug as such, maybe just an occasional loss of synchronization between all the moving parts.

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Someone on the admin side of this forum needs to nuke the spammer (akram009) who posted the comment prior to this one. Seems we have more and more of these idiots popping up lately.