Impossible requests. This includes things like a gendered Ditto, male Froslass, a Level 1 - 10 Solgaleo, etc. The trade would most likely go through if you had one of these Pokemon, but you typically can't legally get them
Names with "YT" or a weird URL are also suspect. This applies to both the Pokemon and the Trainer: if you see something like "AdventurSnivyYT" or a URL, the trade is almost certainly corrupted.
Also note that the Gen 7 GTS is absolutely wrecked -- it freezes no matter what Pokemon you search, so if you want to look at it that way, all GTS trades are broken.