When the input really is the above code, it works fine. What is no doubt happening is that someone did:
[spoiler=Header Yandex Authentication-Results: hotmail.com; spf=pass (sender IP is 87.250.230.162; identity alignment result is pass and alignment mode is relaxed) smtp.mailfrom=libelenna11@yandex.ru; dkim=pass (identity alignment result is pass and alignment mode is relaxed) header.d=yandex.ru; x-hmca=pass header.id=libelenna11@yandex.ru X-SID-PRA: libelenna11@yandex.ru X-AUTH-Result: PASS X-SID-Result: PASS X-Message-Status: n:n X-Message-Delivery: Vj0xLjE7dXM9MTtsPTE7YT0xO0Q9MDtHRD0wO1NDTD0w X-Message-Info: Mwnpjg== Received: from forward20h.cmail.yandex.net ([87.250.230.162]) by BAY004-MC6F31.hotmail.com over TLS secured channel with Microsoft SMTPSVC(7.5.7601.23143); Mon, 2 Nov 2015 02:35:25 -0800 Received: from smtp2h.mail.yandex.net (smtp2h.mail.yandex.net [IPv6:2a02:6b8:0:f05::116]) by forward20h.cmail.yandex.net (Yandex) with ESMTP id 33CFA20BA3 for <XXX@hotmail.com>; Mon, 2 Nov 2015 13:35:23 +0300 (MSK) Received: from smtp2h.mail.yandex.net (localhost [127.0.0.1]) by smtp2h.mail.yandex.net (Yandex) with ESMTP id E598B17007AA for <XXX@hotmail.com>; Mon, 2 Nov 2015 13:35:22 +0300 (MSK) Received: by smtp2h.mail.yandex.net (nwsmtp/Yandex) with ESMTPSA id hULEAclrb1-ZEAa3Ko9; Mon, 2 Nov 2015 13:35:15 +0300 (using TLSv1 with cipher AES128-SHA (128/128 bits)) (Client certificate not present) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=yandex.ru; s=mail; t=1446460522; bh=xTkBiFDrfFJG7jUQkIxvkDq9mDNS19E3ktNxPF6U5/M=; h=Date:From:X-Priority:Message-ID:To:Subject:In-Reply-To:References: MIME-Version:Content-Type; b=wkSm62xTm26AW5so0F3r5YqSAn+L6WDNooDHvyjjQVwk19dGg645QZ5hxe1ntg1DW WplvfMw7z+7yryQj+bnfMINXQJBnAxAtr2R/DGeEjUAxQ6oL7f/rVeFbdPYnBKSRDV qcPSz6pgQJMOXv5O33JKSQnRn0Uvrz7ehVJCHBpg= Authentication-Results: smtp2h.mail.yandex.net; dkim=pass header.i=@yandex.ru X-Yandex-ForeignMX: US Date: Mon, 2 Nov 2015 13:36:25 +0400 From: Elena <libelenna11@yandex.ru> X-Priority: 3 (Normal) Message-ID: <1787761184.20151102133625@yandex.ru> To: XXX Brood <XXX@hotmail.com> Subject: Hallo!!! In-Reply-To: <XXX @phx.gbl> References: <XXX @phx.gbl> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="----------10A1B413D2A0DE219" Return-Path: libelenna11@yandex.ru X-OriginalArrivalTime: 02 Nov 2015 10:35:25.0512 (UTC) FILETIME=[2F4AB480:01D1155A] ------------10A1B413D2A0DE219 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable[/spoiler]
The above code has stuff in it that will break the spoiler tag regex. So YaBB simply spits out the entire string as text.
Edited: And since the YaBBC section of the Spoiler code is the same for 2.6.11 and 2.6.12 I can just about guarantee the same inputted bad code will have the same results.