If you upgrade Zencart from version 1.3.x to V1.5.4, chance is you’ll experience a lot of error on front-end and back-end Admin. Below is the list of files that you need to consider during the manual upgrade. Version 1.5.4 contains new features and bug fixes.

NEW Files ADDED (since release of v1.5.3)

/admin/includes/auto_loaders/config.admin.zcObserverLogEventListener.php (added in v1.5.4)
/admin/includes/auto_loaders/config.admin.zcObserverLogWriterDatabase.php (added in v1.5.4)
/admin/includes/auto_loaders/config.admin.zcObserverLogWriterTextfile.php (added in v1.5.4)
/admin/includes/classes/class.admin.zcObserverLogEventListener.php (added in v1.5.4)
/admin/includes/classes/class.admin.zcObserverLogWriterDatabase.php (added in v1.5.4)
/admin/includes/classes/class.admin.zcObserverLogWriterTextfile.php (added in v1.5.4)
/admin/includes/init_includes/init_cache_key_check.php (added in v1.5.2, updated in 1.5.4)
/includes/classes/ajax/zcAjaxPayment.php (added in v1.5.4)
/includes/templates/template_default/jscript/jquery.min.js (added in v1.5.4)
/includes/templates/template_default/jscript/jscript_framework.php (added in v1.5.4)
/includes/templates/template_default/templates/tpl_ajax_checkout_confirmation_default.php (added in v1.5.4)
/ajax.php (added in v1.5.4)

CHANGED Files (since release of v1.5.3)

/admin/admin_activity.php
/admin/attributes_controller.php
/admin/configuration.php
/admin/countries.php
/admin/coupon_admin.php
/admin/currencies.php
/admin/customers.php
/admin/define_pages_editor.php
/admin/downloads_manager.php
/admin/ezpages.php
/admin/geo_zones.php
/admin/gv_mail.php
/admin/gv_queue.php
/admin/languages.php
/admin/layout_controller.php
/admin/login.php
/admin/media_manager.php
/admin/modules.php
/admin/orders.php
/admin/profiles.php
/admin/salemaker.php
/admin/specials.php
/admin/sqlpatch.php
/admin/store_manager.php
/admin/tax_classes.php
/admin/tax_rates.php
/admin/whos_online.php
/admin/includes/dist-configure.php
/admin/includes/header.php
/admin/includes/auto_loaders/config.admin.zcObserverLogEventListener.php (added in v1.5.4)
/admin/includes/auto_loaders/config.admin.zcObserverLogWriterDatabase.php (added in v1.5.4)
/admin/includes/auto_loaders/config.admin.zcObserverLogWriterTextfile.php (added in v1.5.4)
/admin/includes/classes/class.admin.zcObserverLogEventListener.php (added in v1.5.4)
/admin/includes/classes/class.admin.zcObserverLogWriterDatabase.php (added in v1.5.4)
/admin/includes/classes/class.admin.zcObserverLogWriterTextfile.php (added in v1.5.4)
/admin/includes/classes/upload.php
/admin/includes/functions/admin_access.php
/admin/includes/functions/database.php
/admin/includes/functions/general.php
/admin/includes/functions/html_output.php
/admin/includes/init_includes/init_admin_auth.php
/admin/includes/init_includes/init_admin_history.php
/admin/includes/init_includes/init_cache_key_check.php
/admin/includes/languages/english.php
/admin/includes/languages/english/admin_activity.php
/admin/includes/languages/english/profiles.php
/admin/includes/modules/copy_to_confirm.php
/admin/includes/modules/move_product_confirm.php
/admin/includes/modules/update_product.php
/admin/includes/modules/product_music/copy_to_confirm.php
/admin/includes/modules/product_music/update_product.php
/extras/.htaccess
/extras/curltester.php
/includes/dist-configure.php
/includes/version.php
/includes/classes/ajax/zcAjaxPayment.php (added in v1.5.4)
/includes/classes/class.phpbb.php
/includes/classes/class.smtp.php
/includes/classes/class.zcPassword.php
/includes/classes/payment.php
/includes/classes/shopping_cart.php
/includes/functions/functions_general.php
/includes/languages/english.php
/includes/modules/order_total/ot_coupon.php
/includes/modules/pages/checkout_payment/header_php.php
/includes/modules/pages/checkout_payment/jscript_main.php
/includes/modules/pages/checkout_shipping/header_php.php
/includes/modules/pages/page/header_php.php
/includes/modules/pages/shopping_cart/header_php.php
/includes/modules/payment/authorizenet.php
/includes/modules/payment/authorizenet_aim.php
/includes/modules/payment/authorizenet_echeck.php
/includes/modules/payment/linkpoint_api/class.linkpoint_api.php
/includes/modules/payment/linkpoint_api.php
/includes/modules/payment/paypal.php
/includes/modules/payment/paypaldp.php
/includes/modules/payment/paypalwpp.php
/includes/modules/payment/paypal/paypal_curl.php
/includes/modules/shipping/storepickup.php
/includes/templates/template_default/common/html_header.php
/includes/templates/template_default/jscript/jquery.min.js (added in v1.5.4)
/includes/templates/template_default/jscript/jscript_framework.php (added in v1.5.4)
/includes/templates/template_default/templates/tpl_ajax_checkout_confirmation_default.php (added in v1.5.4)
/includes/templates/template_default/templates/tpl_checkout_payment_default.php
/ajax.php (added in v1.5.4)
/zc_install/ … assume *all* of /zc_install/ has changed

And here are all the locations of error may be displayed on frontpage.