I am an amateur map maker, and it really annoys me when I see mobs that spawn from spawners spawning in walls, then suffocating. I've attached a screenshot to show you guys. The spawner is a spawner created in MCEdit, and also spawns a custom mob stacked entity.
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Is it a custom spawner? If it is, is any other entity riding that skeleton? If not, it is, like SteveNL2000 pointed out, definitely the return of the most annoying bug of the year.

looks like a custom spawner (the custom equipment they spawn with)

Yeah, it is. Is it the fact that it's a custom spawner that makes mobs spawn in walls?

https://bugs.mojang.com/browse/MC-57831?focusedCommentId=200862&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-200862.
I am not sure wether this is a duplicate or not, but I don't want to trust a single person saying it's not.

Stacked entities follow the spawning conditions of the topmost entity, so if you for example have a splash potion riding a skeleton, it will spawn inside walls and in any light level. I don't know what the spawner in the image is trying to spawn, so I cannot tell if it's the case here.

Well, if it is created with mcedit, it probably also entered the posision data

Invalid as long as this cannot be reproduced in vanilla Minecraft without MCEdit.

this cannot be reproduced in vanilla Minecraft without MCEdit
I am not so sure about that. Almost everything, if not everything MCEdit can add to a spawner can now be done using /setblock and /blockdata. Is there any proof that this can only occur with MCEdit?

No proof, but without having the appropriate setblock / blockdata command(s) we cannot validate that.
Duplicate of MC-57831. This issue indicates that that bug has returned in 1.8.