diff --git a/docs/CONFIGURATION.md b/docs/CONFIGURATION.md
index f531be4cc..a9034f5cd 100644
--- a/docs/CONFIGURATION.md
+++ b/docs/CONFIGURATION.md
@@ -661,6 +661,12 @@ for time-based filtering. An introduction to the CEL language can be found
 are provided in the [language definition][CEL lang] as well as in the
 [extension documentation][CEL ext].
 
+**NOTE:** Expressions that may be valid and compile, but fail at runtime will
+result in the expression reporting as `true`. The metrics will pass through
+as a result. An example is when reading a non-existing field. If this happens,
+the evaluation is aborted, an error is logged, and the expression is reported as
+`true`, so the metric passes.
+
 > NOTE: As CEL is an *interpreted* languguage, this type of filtering is much
 > slower compared to `namepass`/`namedrop` and friends. So consider to use the
 > more restricted filter options where possible in case of high-throughput