If Open Source software and proprietary own developments are contained in a product and own license terms shall be used for the proprietary developments, care must be taken to ensure that the own license terms are not in conflict with the Open Source license terms.
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No, this is not necessary. The requirements of all relevant Open Source licenses are met if a copy of the license text is delivered to the customer together with the software. The own license terms for possible proprietary components do not have to contain the license texts again, but at the same time must not be in contradiction with them (see FAQ "What has to be noted in the case of license terms for own developments if distributed in combination with free software?")

Open Source licenses deal with the question if the license text has to be supplied with the product in paper form or as a file, or whether it is sufficient to specify a URL differently. Most licenses, like the GPL, require that the license text is supplied together with the product. The Landgericht München I (Regional Court Munich), has explicitly deemed this as necessary in a judgment (http://www.ifross.de/Fremdartikel/LGMuenchenUrteil.pdf).
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If code components that were licensed under different license conditions are compiled to form a new work, a question arises: Is the common use of the code permissible under license law? If for example code A under license X is linked with code B under license Y to form a new program, licenses X and Y must permit this. If this is the case, license compatibility exists. 

License compatibility is particularly important for copyleft licenses (→ What types of licenses are there for open-source software, and how do they differ?) since licenses such as the GPL provide that derived works may only be used under the same license conditions (that is, those of the GPL). Hence copyleft licenses are only compatible with other open source licenses when one of the following conditions is met: » Weiter

The precise time of the conclusion of the license agreement is important since from this time on, the provisions of the open source license become effective with their associated rights and obligations. The license agreements are regularly added to the software in an independent file, generally termed "license" or "copying".  If the user receives the software including the license file, this does not constitute a license agreement.  According to German contractual law, two corresponding declarations of intent are required, the "offer" and "acceptance."  In addition, the contents of the agreement must be clearly identifiable for an agreement to be effectively concluded. 
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The GNU General Public License (GPL) is the most important and widespread open-source license.  Approximately 60% of all open-source software is under this license.  This includes well-known programs like Linux or Busybox. The GPL is the template for all licenses with a strict copyleft as well as version 2 (→ GNU General Public License, Version 2) and version 3 (→ The GNU General Public License, Version 3).

The GNU Lesser General Public License (LGPL) was earlier termed the "GNU Library General Public License" and was especially conceived for program libraries.  It has a restricted copyleft that enables the libraries to be linked with open-source software and proprietary software. » Weiter

The rights of use granted in the countless Open Source licenses (http://www.ifross.eu/en/license-center) only differ slightly (→ How can I use open source software? What rights of use to I acquire?), but there can be substantial differences in the required terms of the license.  Although the licensee may not request any license fees, licensees are commonly subject to other requirements.
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