Compile errors caused by an actor calling a function with something hooked up to a removed parameter can cause that actor to lose it's place in the hierarchy. Typically, the actor will be fixed once the node is fixed up and the blueprint successfully compiles. However, if the level is saved while it's in that state and checked into source control, the actor data can be permanently lost. 

Linked UDN provides a video and example project.

Steps to Reproduce
  1. Create a blueprint function library that exposes a function with 2+ parameters
  2. In an actor blueprint, call that function. Save everything and close the editor
  3. Remove one of the parameters from the blueprint function. Compile and reopen the editor
  4. Open the actor that called the function and note the error pin. Hook something up to the parameter that is still valid and compile/save. Note that the actor loses it's parent, and won't be fixed until it can once again compile.

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ComponentGameplay - Blueprint
Affects Versions4.20
Target Fix4.21
Fix Commit4454677
Main Commit4454809
Release Commit4454677
CreatedOct 4, 2018
ResolvedOct 11, 2018
UpdatedOct 15, 2018