Despite the fact that the Parsha of Vayakhel deals entirely with the Mishkan, interestingly enough the Parsha begins by cautioning Bnei Yisrael to heed the mitzvah of Shabbos and not to perform any of the forbidden melachos. Rashi explains the reason behind the Torah's juxtaposition of this warning to the portion that describes the building of the Mishkan, as follows: Although Hashem commanded Bnei Yisrael to build the Bais HaMikdosh, nevertheless, they were to be careful not to build it on Shabbos for that would be a desecration of the holy day.
Rav Wolbe (Shiurei Chumash) comments that it is difficult to understand why Bnei Yisrael would have even entertained such a thought that the building of the Mishkan should override the prohibition of performing melachah on Shabbos - that the Torah was compelled to negate this possibility. We don't find the Torah warning us that even though there is a mitzvah to write a sefer Torah, bear in mind that it is forbidden to write it on Shabbos. Moreover, we know that a special pasuk is always required to allow any mitzvah to override the prohibition of performing melachah on Shabbos. Why would Bnei Yisrael think that the mitzvah of building the Mishkan is any different?
He continues that although he doesn't have a clear cut answer to this question, nevertheless, he wishes to offer an insight that contains a lesson for life. We just read last week in Parshas Ki Sisa how Bnei Yisrael sinned by making the golden calf. The Ramban explains that their transgression was a result of a mistake. Moshe Rabbeinu was the leader and guiding light of Bnei Yisrael. When forty days passed and Moshe had not returned, they were thrown into a panic: who would guide them and connect them with their Creator? They created the golden calf to fulfill that role. Their intentions were noble, but that did not justify their actions which could be termed a mitzvah haba'ah b'aveirah - a mitzvah that came through a sin. Therefore, the Torah warns Bnei Yisrael: Even though I am commanding you to build a Mishkan which is to act as an abode for Hashem and a means of connecting to Him, it should not be built on Shabbos for this would constitute a mitzvah haba'ah b'aveirah.
This concept formed one of the focal points of Reb Yisrael Salanter's teachings. A person must ensure that his mitzvos not be performed by way of aveiros. He would depict a scene where a Maggid came to town to deliver a mussar shmuess (discourse). Everyone in the town was interested in hearing his words of wisdom, and they rushed to the shul where the shmuess would be held. Due to the rush and bedlam, one person knocked over a passerby, and another got angry because someone cut him off, and so on. Yes, they were running to perform a mitzvah - to hear words that will help them improve their avodas Hashem - but at what expense? There is another well known story of Reb Yisrael Salanter that illustrates this point. One morning before Shachris, a man put on his tallis and wrapped his face in its folds. However, as he threw the tzitzis over his shoulder the strings slapped Reb Yisrael in his face! Here too, the man's mitzvah came about through a flaw in his bein adom l'chaveiro.
A practical application of Reb Yisrael Salanter's moshel, would include double parking to enable one to come on time to Mincha or some other mitzvah. One of the benefits of regular mussar study is that it gives one the ability to perceive his actions in a different, more objective light. Even without opening a mussar sefer, it behooves us to take a few minutes to contemplate our actions to ensure that out mitzvos are just mitzvos, and not chas v'shalom tainted with "small" transgressions.