There's a small dark blob or coloured spot in my projected image that moves with the picture. What is it?

First localise it: pause on a plain white image, then look at what the artefact does. That one test tells you which of three things you have.

A soft dark blob that stays fixed on the screen regardless of content, and does not sharpen or move when you adjust focus: dust in the optical path, a "dust blob". It has settled on an internal surface, typically the imaging chip's cover glass or a lens element inside the light engine, and it casts a shadow onto every frame.

A sharp, single-pixel dot of constant colour that never changes: a stuck or dead pixel on the imaging chip. On a DLP that is one mirror stuck; on an LCD-family projector, one pixel on one panel, which is why it has a colour.

A smudge that changes shape or edge sharpness as you rack the focus: dirt on the lens front element. Fingerprints, cooking film, a moth's contribution. This one is yours to fix.

What to do about each. Exterior lens cleaning is fair game: a blower first, then a proper lens cloth or lens-cleaning solution on the front element, gently, no household glass cleaner. Anything internal is not. Opening the light path yourself adds more dust than it removes and can void the warranty; the service centre works in a cleaner room than your living room, and it has the tools to reseat things you cannot.

Dust blobs are a known serviceable condition and authorised service deals with them routinely. Stuck pixels may be covered under warranty depending on the brand's pixel policy; the manufacturers publish stated terms on how many defective pixels count as a fault, so check the manufacturer's page for your model.

Either way, go to authorised service with photos of the artefact on a plain white screen, ideally one wide shot and one close, taken with the room lights off. That documentation speeds the claim considerably, because the technician can see it before the projector arrives. Send us the same photos and the model and we can tell you which of the three it is before you pack anything.