If an API query is made with the deprecated rctoken parameter and without comment or parsedcomment in rcprop, it logs a warning (or throws an exception, depending on the value for $wgCommentTableSchemaMigrationStage):
2017-09-07 16:54:53 error WARNING: [b09bca563c5d638aeea2e18a] /w/api.php?action=query&list=recentchanges&rcprop=&rctoken=patrol ErrorException from line 309 of /usr/local/src/MediaWiki/core/includes/debug/MWDebug.php: PHP Warning: Missing rc_comment_text and rc_comment_data fields in row with MIGRATION_OLD [Called from CommentStore::getCommentInternal in /usr/local/src/MediaWiki/core/includes/CommentStore.php at line 203] {"exception_id":"b09bca563c5d638aeea2e18a","caught_by":"mwe_handler"}
[Exception ErrorException] (/usr/local/src/MediaWiki/core/includes/debug/MWDebug.php:309) PHP Warning: Missing rc_comment_text and rc_comment_data fields in row with MIGRATION_OLD [Called from CommentStore::getCommentInternal in /usr/local/src/MediaWiki/core/includes/CommentStore.php at line 203]
#0 [internal function]: MWExceptionHandler::handleError(integer, string, string, integer, array)
#1 /usr/local/src/MediaWiki/core/includes/debug/MWDebug.php(309): trigger_error(string, integer)
#2 /usr/local/src/MediaWiki/core/includes/debug/MWDebug.php(164): MWDebug::sendMessage(string, array, string, integer)
#3 /usr/local/src/MediaWiki/core/includes/GlobalFunctions.php(1203): MWDebug::warning(string, integer, integer, string)
#4 /usr/local/src/MediaWiki/core/includes/CommentStore.php(203): wfLogWarning(string)
#5 /usr/local/src/MediaWiki/core/includes/CommentStore.php(334): CommentStore->getCommentInternal(Wikimedia\Rdbms\DatabaseMysqli, array, boolean)
#6 /usr/local/src/MediaWiki/core/includes/changes/RecentChange.php(950): CommentStore->getCommentLegacy(Wikimedia\Rdbms\DatabaseMysqli, stdClass, boolean)
#7 /usr/local/src/MediaWiki/core/includes/changes/RecentChange.php(121): RecentChange->loadFromRow(stdClass)
#8 /usr/local/src/MediaWiki/core/includes/api/ApiQueryRecentChanges.php(574): RecentChange::newFromRow(stdClass)
#9 /usr/local/src/MediaWiki/core/includes/api/ApiQueryRecentChanges.php(393): ApiQueryRecentChanges->extractRowInfo(stdClass)
#10 /usr/local/src/MediaWiki/core/includes/api/ApiQueryRecentChanges.php(133): ApiQueryRecentChanges->run()
#11 /usr/local/src/MediaWiki/core/includes/api/ApiQuery.php(253): ApiQueryRecentChanges->execute()
#12 /usr/local/src/MediaWiki/core/includes/api/ApiMain.php(1583): ApiQuery->execute()
#13 /usr/local/src/MediaWiki/core/includes/api/ApiMain.php(546): ApiMain->executeAction()
#14 /usr/local/src/MediaWiki/core/includes/api/ApiMain.php(517): ApiMain->executeActionWithErrorHandling()
#15 /usr/local/src/MediaWiki/core/api.php(83): ApiMain->execute()
#16 {main}