diff --git a/static/css/exception.css b/static/css/exception.css
new file mode 100644
index 0000000..1f141c5
--- /dev/null
+++ b/static/css/exception.css
@@ -0,0 +1,78 @@
+div.debugger { text-align: left; padding: 12px; margin: auto;
+               background-color: white; }
+div.detail { cursor: pointer; }
+div.detail p { margin: 0 0 8px 13px; font-size: 14px; white-space: pre-wrap;
+               font-family: monospace; }
+div.explanation { margin: 20px 13px; font-size: 15px; color: #555; }
+div.footer   { font-size: 13px; text-align: right; margin: 30px 0;
+               color: #86989B; }
+
+h2           { font-size: 16px; margin: 1.3em 0 0.0 0; padding: 9px;
+               background-color: #11557C; color: white; }
+h2 em, h3 em { font-style: normal; color: #A5D6D9; font-weight: normal; }
+
+div.traceback, div.plain { border: 1px solid #ddd; margin: 0 0 1em 0; padding: 10px; }
+div.plain p      { margin: 0; }
+div.plain textarea,
+div.plain pre { margin: 10px 0 0 0; padding: 4px;
+                background-color: #E8EFF0; border: 1px solid #D3E7E9; }
+div.plain textarea { width: 99%; height: 300px; }
+div.traceback h3 { font-size: 1em; margin: 0 0 0.8em 0; }
+div.traceback ul { list-style: none; margin: 0; padding: 0 0 0 1em; }
+div.traceback h4 { font-size: 13px; font-weight: normal; margin: 0.7em 0 0.1em 0; }
+div.traceback pre { margin: 0; padding: 5px 0 3px 15px;
+                    background-color: #E8EFF0; border: 1px solid #D3E7E9; }
+div.traceback .library .current { background: white; color: #555; }
+div.traceback .expanded .current { background: #E8EFF0; color: black; }
+div.traceback pre:hover { background-color: #DDECEE; color: black; cursor: pointer; }
+div.traceback div.source.expanded pre + pre { border-top: none; }
+
+div.traceback span.ws { display: none; }
+div.traceback pre.before, div.traceback pre.after { display: none; background: white; }
+div.traceback div.source.expanded pre.before,
+div.traceback div.source.expanded pre.after {
+    display: block;
+}
+
+div.traceback div.source.expanded span.ws {
+    display: inline;
+}
+
+div.traceback blockquote { margin: 1em 0 0 0; padding: 0; white-space: pre-line; }
+div.traceback img { float: right; padding: 2px; margin: -3px 2px 0 0; display: none; }
+div.traceback img:hover { background-color: #ddd; cursor: pointer;
+                          border-color: #BFDDE0; }
+div.traceback pre:hover img { display: block; }
+div.traceback cite.filename { font-style: normal; color: #3B666B; }
+
+pre.console { border: 1px solid #ccc; background: white!important;
+              color: black; padding: 5px!important;
+              margin: 3px 0 0 0!important; cursor: default!important;
+              max-height: 400px; overflow: auto; }
+pre.console form { color: #555; }
+pre.console input { background-color: transparent; color: #555;
+                    width: 90%; font-family: 'Consolas', 'Deja Vu Sans Mono',
+                    'Bitstream Vera Sans Mono', monospace; font-size: 14px;
+                     border: none!important; }
+
+span.string { color: #30799B; }
+span.number { color: #9C1A1C; }
+span.help   { color: #3A7734; }
+span.object { color: #485F6E; }
+span.extended { opacity: 0.5; }
+span.extended:hover { opacity: 1; }
+a.toggle { text-decoration: none; background-repeat: no-repeat;
+           background-position: center center;
+           background-image: url(?__debugger__=yes&cmd=resource&f=more.png); }
+a.toggle:hover { background-color: #444; }
+a.open { background-image: url(?__debugger__=yes&cmd=resource&f=less.png); }
+
+div.traceback pre, div.console pre {
+    white-space: pre-wrap;       /* css-3 should we be so lucky... */
+    white-space: -moz-pre-wrap;  /* Mozilla, since 1999 */
+    white-space: -pre-wrap;      /* Opera 4-6 ?? */
+    white-space: -o-pre-wrap;    /* Opera 7 ?? */
+    word-wrap: break-word;       /* Internet Explorer 5.5+ */
+    _white-space: pre;           /* IE only hack to re-specify in
+                                 addition to word-wrap  */
+}
diff --git a/templates/base.html b/templates/base.html
new file mode 100644
index 0000000..8452f57
--- /dev/null
+++ b/templates/base.html
@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<meta http-equiv="X-UA-Compatible" content="IE=edge">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<title>{% block title %}{% endblock %}</title>
+
+  <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-T3c6CoIi6uLrA9TneNEoa7RxnatzjcDSCmG1MXxSR1GAsXEV/Dwwykc2MPK8M2HN" crossorigin="anonymous">
+
+{% block style %}
+{% endblock %}
+</head>
+
+<body>
+{% block content %}
+{% endblock %}
+
+{% block scripts %}
+{% endblock %}
+</body>
+</html>
diff --git a/templates/show_error.html b/templates/show_error.html
new file mode 100644
index 0000000..dd79c3c
--- /dev/null
+++ b/templates/show_error.html
@@ -0,0 +1,23 @@
+{% extends "base.html" %}
+
+{% block style %}
+<link rel="stylesheet" href="{{url_for('static', filename='css/exception.css')}}" />
+{% endblock %}
+
+{% block content %}
+<div class="p-2">
+
+<h1>Software error: {{ exception_type }}</h1>
+<div>
+  <pre>{{ exception }}</pre>
+</div>
+
+<h2 class="traceback">Traceback <em>(most recent call last)</em></h2>
+{{ summary | safe }}
+
+<p>Error in function "{{ last_frame.f_code.co_name }}": {{ last_frame_args | pprint }}</p>
+<pre>{{ last_frame.f_locals | pprint }}</pre>
+
+</div>
+
+{% endblock %}
diff --git a/web_view.py b/web_view.py
index a76ca5c..1001d2d 100755
--- a/web_view.py
+++ b/web_view.py
@@ -2,16 +2,48 @@
 
 """Web page to show upcoming events."""
 
+import inspect
+import sys
+import traceback
 from datetime import date, datetime, timezone
 
-from flask import Flask, render_template
+import flask
+import werkzeug
+import werkzeug.debug.tbtools
 
 from agenda import get_data
 
-app = Flask(__name__)
+app = flask.Flask(__name__)
 app.debug = True
 
 
+@app.errorhandler(werkzeug.exceptions.InternalServerError)
+def exception_handler(e: werkzeug.exceptions.InternalServerError) -> tuple[str, int]:
+    """Handle exception."""
+    exec_type, exc_value, current_traceback = sys.exc_info()
+    assert exc_value
+    tb = werkzeug.debug.tbtools.DebugTraceback(exc_value)
+
+    summary = tb.render_traceback_html(include_title=False)
+    exc_lines = "".join(tb._te.format_exception_only())
+
+    last_frame = list(traceback.walk_tb(current_traceback))[-1][0]
+    last_frame_args = inspect.getargs(last_frame.f_code)
+
+    return (
+        flask.render_template(
+            "show_error.html",
+            plaintext=tb.render_traceback_text(),
+            exception=exc_lines,
+            exception_type=tb._te.exc_type.__name__,
+            summary=summary,
+            last_frame=last_frame,
+            last_frame_args=last_frame_args,
+        ),
+        500,
+    )
+
+
 @app.route("/")
 def index() -> str:
     """Index page."""
@@ -27,7 +59,7 @@ def index() -> str:
     def days(when: date) -> str:
         return "today" if when == today else f"{(when - today).days:,d} days"
 
-    return render_template("index.html", days=days, days_hours=days_hours, **data)
+    return flask.render_template("index.html", days=days, days_hours=days_hours, **data)
 
 
 if __name__ == "__main__":