A couple of customers already ran into this issue: if you created your farm based on the SharePoint 2013 SP1 slipstream build available on MSDN and VLSC you might not be able to install the latest patches (April CU and PU). The reason is that starting in April the RTM baseline is no longer supported by the fixes. The fixes require the SP1 baseline.
But there are two flavors of SP1: the original one with version 15.0.4569.1506 and a rereleased version with build 15.0.4571.1502. The slipstream builds never got rereleased as they were not affected by the problems that led to the rerelease of SP1.
The problem with April fixes is that they only install on the patch baseline of the rereleased SP1:
<TargetVersion Validate="true" ComparisonType="Equal" ComparisonFilter="MajorMinorUpdate">15.0.4571.1502</TargetVersion>
A workaround right now is to install the rereleased SP1 before applying the April fixes.
Be aware that this also applies to each installed language pack.
Update April 20th, 2015: You can use the script to identify the patch baseline version info to see which components need upgrade.
[Update from May 12th, 2015]: May CU solves the problem with the different SP1 baselines introduced in April CU. May CU can be installed as well on the old and on the new SP1 baseline!