I can't speak as to why 'they' specifically think it's the case - the most straightforward answer is that they're making a joke, but I can tell you that bad dumps resulting in visual artifacts is very plausible. It doesn't even have to be a bad dump:
The Amiga game Pinkie is spread across three disks, with five worlds you can play in any order. The graphics and scripting necessary to show the ending is on all three disks to save you a disk swap at the end. Due to an oversight during mastering, the file containing the background graphics to be used in the ending was misnamed, but only on Disks 2 and 3. As a result, when you win Pinkie or cheat to view the ending while on any world other than the first, the ending looks like this:
Whereas, if the ending is viewed from Disk 1, the ending looks like this:
The animation itself (not to spoil it) plays as normal regardless. This isn't due to a bad dump, this is the behaviour of the actual, retail disks. More information on English Amiga Board.
If an error in a dump or a master is in part of the game that isn't executed or used as part of a branch or calculation, i.e. it's just dumb data that's copied from place to place and displayed or played as sound, then there could be glitchy or discoloured tiles, missing samples, strangely placed enemies, or any number of effects. As discussed in this question about Amiga error handling, Amiga games sometimes just barrel on through.
The standard Sega Genesis header contains a checksum of the game image at $18E, but it isn't verified on boot. If you made a reproduction cartridge of a downloadable game like Tanglewood but for the sake of argument the download was bad or the upload into fooPROM was bad, you could get adverse visual effects playing on hardware, yes. (The Master System BIOS may check before booting though and you'll get
On the Commodore 64, depending on your machine condition, tape condition and volume etc., a game like Impossible Mission can seemingly load just fine from cassette, but may develop errors later in the room layouts preventing you from advancing with invisible walls or intangible floors. Loading a game from cassette tape into memory is very much like making a dump of a cart, except much more unreliable!
Cracked Amiga games can be missing levels, deliberately or inadvertently, or be suffering from any number of effects from copy-protection (whether removed, not-removed, partially removed...)