How to reproduce:
1. Create a new project
2. Add GameObject to the scene
3. Select it and add a Tag via Inspector (example "Tag1")
4. Assign Tag ("Tag1") to GameObject
5. Create a script which validates a given Tag name and attach it to GameObject
-if you build the scene at this moment, it should find the tag
6. Create another Tag and assign it to the GameObject (for example "Tag2")
7. Delete "Tag1" from Tag Manager
8. Update the script to validate the new tag "Tag2"
9. Play the scene and notice that it finds the Tag
10. Build and run Standalone Player/ Android/ iOS

Result: you will get UnityException: GameObject has undefined tag! error

Reproducible: 5.5.0f1, 5.5.2p3, 5.6.0f1, 2017.1.0a3. (Reproduced on windows standalone player, Android and iOS)

New Tags' system was introduces in Unity 5.5
Workaround is to restart editor before you make a build if you remove one or more tags.