See PHI1711. apcu_store() may raise a notice like this:
apcu_store(): GC cache entry "<some-key-name>" was on gc-list for 3601 seconds
After D21044, these can be user-visible fatals.
In theory, I think this is supposed to indicate a sort of memory leak (an entry should have been GC'd earlier, but was not GC'd by the normal GC). In practice, this condition seems to be routine and unconerning, and APCu decreased its severity at some point, e.g.:
The most practical response seems to be to use @ to silence the notice.