# Buggy code
def dispatch_error(self, update, error):
    """Dispatches an error.

    Args:
        update (:obj:`str` | :class:`telegram.Update` | None): The update that caused the error
        error (:obj:`Exception`): The error that was raised.

    """
    if self.error_handlers:
        for callback in self.error_handlers:
            if self.use_context:
                callback(update, CallbackContext.from_error(update, error, self))
            else:
                callback(self.bot, update, error)

    else:
        self.logger.exception(
            "No error handlers are registered, logging exception.", exc_info=error
        )


# Traceback (telegram.error.BadRequest)
<project_path>/venv/bin/python <project_path>/issue.py
2017-06-20 23:24:11,743 - __main__ - INFO - /start
2017-06-20 23:24:11,906 - telegram.ext.dispatcher - WARNING - A TelegramError was raised while processing the Update.
2017-06-20 23:24:11,907 - __main__ - ERROR - Boom!
2017-06-20 23:24:11,907 - __main__ - INFO - /async_start
2017-06-20 23:24:12,188 - telegram.utils.promise - ERROR - An uncaught error was raised while running the promise
Traceback (most recent call last):
File "<project_path>/venv/src/python-telegram-bot/telegram/utils/promise.py", line 42, in run
self._result = self.pooled_function(*self.args, **self.kwargs)
File "<project_path>/issue.py", line 25, in action_async_start
update.message.reply_text(msg, parse_mode=ParseMode.HTML)
File "<project_path>/venv/src/python-telegram-bot/telegram/message.py", line 340, in reply_text
return self.bot.send_message(self.chat_id, *args, **kwargs)
File "<project_path>/venv/src/python-telegram-bot/telegram/bot.py", line 125, in decorator
result = func(self, *args, **kwargs)
File "<project_path>/venv/src/python-telegram-bot/telegram/bot.py", line 158, in decorator
return Bot._message_wrapper(self, url, data, *args, **kwargs)
File "<project_path>/venv/src/python-telegram-bot/telegram/bot.py", line 146, in _message_wrapper
result = self._request.post(url, data, timeout=kwargs.get('timeout'))
File "<project_path>/venv/src/python-telegram-bot/telegram/utils/request.py", line 252, in post
**urlopen_kwargs)
File "<project_path>/venv/src/python-telegram-bot/telegram/utils/request.py", line 194, in _request_wrapper
raise BadRequest(message)
telegram.error.BadRequest: Can't parse entities in message text: unsupported start tag "invalid_tag" at byte offset 0

Process finished with exit code 0

# Fixed code
def dispatch_error(self, update, error, promise=None):
    """Dispatches an error.

    Args:
        update (:obj:`str` | :class:`telegram.Update` | None): The update that caused the error
        error (:obj:`Exception`): The error that was raised.
        promise (:class:`telegram.utils.Promise`, optional): The promise whose pooled function
            raised the error.

    """
    async_args = None if not promise else promise.args
    async_kwargs = None if not promise else promise.kwargs

    if self.error_handlers:
        for callback, run_async in self.error_handlers.items():
            if self.use_context:
                context = CallbackContext.from_error(
                    update,
                    error,
                    self,
                    async_args=async_args,
                    async_kwargs=async_kwargs,
                )
                if run_async:
                    self.run_async(callback, update, context, update=update)
                else:
                    callback(update, context)
            else:
                if run_async:
                    self.run_async(callback, self.bot, update, error, update=update)
                else:
                    callback(self.bot, update, error)

    else:
        self.logger.exception(
            "No error handlers are registered, logging exception.", exc_info=error
        )