Currently, the Schematron rules for DB 5.0 constrain the use of admonitions (caution, note, tip, etc) as descendants of table. Aside from the impracticality of this constraint for technical writers (who frequently use admonitions in tables cells), this rule seems to cause inconsistent validation in some tools (for example, Oxygen 13.2, which sometimes flags the condition and sometimes doesn't).
See lines 216-226 in docbook.sch.
+1
I've always wondered why that one is in there (and have removed it from our local oxygen framework).
Restriction against admonitions in formal objects (tables, figures, examples) removed in 5.1