There are plenty of mistakes you can make, Google Ads is a highly technical tool. Here are my top 3, with detail around them:

Wrong keyword matches. The most fundamental issue in Google Ads is using the right keywords and the right match types. There are 4 keyword match types. Broad, ‘+Broad +match +modifier’ (BMM), “phrase” & [exact]. Some people also call -negative a match type. There are different strategies for using match types. Some people use a combination of BMM with negatives. Others like to use phrase only. Others like to build out single keyword adgroups (SKAGS), using only exact match. All these strategies are legitimate. The big mistake is to use broad match keywords. There are limited circumstances where you might want to use a broad keyword, but in the vast majority of cases, you should steer clear from broad. It allows Google to match synonyms and related words which are not necessarily relevant.

Using combined Search & Display campaigns. This happens a lot because this is the default setting on a new campaign. When you hit create new campaign, Google provides you a campaign that will target both the Search and Display Networks. This is a big issue because these are very different networks and should be used differently. Search, is usually where you start and allows you to target users based on the keywords they have searched for. Search only allows text ads. Most often search is best for low marketing funnel targeting. Display on the other hand, uses Google’s display network, which places your ad in a placement on their network of 3rd party websites (think adsense). This is a very different type of ad and targeting. The GDN should be reserved for higher funnel activity such as raising awareness and branding. In a combined campaign, Google will take your keywords and try and build an audience for GDN based on it, this is extremely loose targeting & not recommended. Furthermore, GDN allows you to run image ads which will perform better then text ads on that network. For all these reasons, these campaigns should be separated out. They have very different targeting, creative, intent & goals and should be setup accordingly.

Auto-implementing Google suggestions. This is another issue that happens by default and most people don’t even know about it. Google Ads provides recommendations for campaign improvement. The problem is that they are computer generated, while sometimes they are helpful, they are often not perfectly relevant and can make suggestions that are in Google’s interest and not necessarily yours. For example, they often recommend increasing budgets or expanding campaign keywords that are not relevant. The second issue is that by default, the auto-implementation is turned on and will be applied automatically after 14 days whether reviewed or not reviewed. This can result in changes occurring in your account that are detrimental to your campaign and cause you to spend more money without return