diff --git a/lib/editor/util/validation.js b/lib/editor/util/validation.js
index 486849006..dcb73bf70 100644
--- a/lib/editor/util/validation.js
+++ b/lib/editor/util/validation.js
@@ -1,5 +1,5 @@
 // @flow
-
+import clone from 'lodash.clonedeep'
 import moment from 'moment'
 import validator from 'validator'
 
@@ -22,7 +22,6 @@ export function validate (
   const isRequiredButEmpty = required && doesNotExist(value)
   let reason = 'Required field must not be empty'
   const agencies = getTableById(tableData, 'agency')
-  const scheduleExceptions = getTableById(tableData, 'scheduleexception')
   switch (inputType) {
     case 'GTFS_ID':
       const indices = []
@@ -190,24 +189,24 @@ export function validate (
       return false
     case 'EXCEPTION_DATE': // a date cannot be selected more than once (for all exceptions)
       const dateMap = {}
-      // FIXME: this declaration of exceptions is redundant
-      const exceptions: Array<ScheduleException> = (scheduleExceptions: Array<ScheduleException>)
+      // Clone exceptions to avoid mutating table in store.
+      const scheduleExceptions: Array<ScheduleException> = clone(getTableById(tableData, 'scheduleexception'))
       if (entity) {
-        const exceptionIndex = exceptions.findIndex(se => se.id === entity.id)
+        const exceptionIndex = scheduleExceptions.findIndex(se => se.id === entity.id)
         if (exceptionIndex !== -1) {
-          exceptions.splice(exceptionIndex, 1)
+          scheduleExceptions.splice(exceptionIndex, 1)
         }
         const castedScheduleException: ScheduleException = ((entity: any): ScheduleException)
-        exceptions.push(castedScheduleException)
+        scheduleExceptions.push(castedScheduleException)
       }
 
-      for (let i = 0; i < exceptions.length; i++) {
-        if (exceptions[i].dates) {
-          exceptions[i].dates.map(d => {
+      for (let i = 0; i < scheduleExceptions.length; i++) {
+        if (scheduleExceptions[i].dates) {
+          scheduleExceptions[i].dates.map(d => {
             if (typeof dateMap[d] === 'undefined') {
               dateMap[d] = []
             }
-            dateMap[d].push(exceptions[i].id)
+            dateMap[d].push(scheduleExceptions[i].id)
           })
         }
       }