Glitchy or green-screen video: three causes of "plays but broken"

Old hardware decoders work within a declared tier: the box says "supports H.264", but the chip only reaches a certain Level — feed it a 4K30 or high-fps 1080p file and the symptom is not an error message but blocks, green screens, and desync. Profile limits behave the same: chips that only know Baseline/Main choke on High-profile tools.

Diagnosis: use the codec checker to read the file Profile and Level and compare against device specs — old TVs, set-top boxes, and car units are the usual victims. The fix is clear: re-encode to Baseline/Main with a Level and resolution the device can handle.

Interrupted transfers, incomplete copies, and bad storage blocks all shave bytes off the file — when the damage lands near a keyframe, the picture stays broken from that frame until the next usable one. The signature: glitching at a fixed position, starting at one point and persisting, identical on every device.

There is no true "repair" for damaged data: the lost bytes are gone, and re-encoding just bakes the mess in. The right move is re-sourcing — export again, download again, copy from the original device again. This is why masters always keep an original copy.

When merged segments differ in encoding parameters (different devices, settings, or recording dates), stream-copy concatenation glitches at the seams — each segment plays fine alone, the joined file breaks at the joint. That is the classic seam signature.

Fix: switch to re-encoding merge, where the encoder normalizes all segments onto one parameter set and the seams come out clean. Slower than stream copy, but it is the only correct path when parameters differ. Comparing segment parameters with the checker before merging predicts the risk in advance.