Mistake No 1
Not showing your users the correct keyboard
Give users the right keyboard on mobile forms, typing is easy when users do it on a desktop. But it's to do on mobile device because not all the keys are visible on the keyboard at once. A small screen can only display a limited set of keys.
Most mobile operating systems have designed separate keyboard optimized for specific form field inputs. These optimized keyboards are there to use, but most User Experience designer fails to test and guide their developer on triggering the correct keyboard.
Example
iOS keyboard
There are mainly 4 types of a keyboard that can be triggered for different types of form fields, please see as an example the iOS keyboard option below.
Mistake No 2
Not understanding the importance of mobile button placement
Button placement affects how fast users complete their tasks, placing your call to action where users expect to find them leads to faster task completion. When the user gets more done in less time, they're more satisfied.
The first location to decide on is whether to place your call to action button at the top or bottom of the screen. Most users start by scanning the content first because it relates to their task. Their eyes move from the top half of the screen towards the bottom. When the content ends, they're looking for an action.
Mistake No 3
Using too many text button
The usability standards for button are higher for mobile apps than desktop apps. It's easy to design text buttons on your interface because they don't take much effort to design. But the consequences of doing this are frustrated users who have trouble reading, recognizing, and tapping your button. Do not take the lazy way out and abuse text buttons. Your call to actions should always look and feel like buttons
Example
Text buttons are harder to tap
Text buttons are harder to tap because a finger is larger than a mouse cursor. Placing it over a smaller target feels awkward for users. Their finger covers the text of the button with no visual cue to confirm if the action registered.
Text buttons are harder to recognize
The difference between text buttons and text color. it makes text buttons difficult to recognize. Without a unique shape, users are more likely to overlook text buttons and miss the call to action.
Take away
Make sure you serve the correct keyboard on your form fields
Understand the importance of buttons placement within the user journey, do not make your user think
When you having a primary and a secondary CTA, always place primary action to the right which is more natural on the reading flow
Try to limit the usage of text buttons as they make look nice but can be anti-user friendly