May be user error here, but maybe not. Using ECMA2015 + babel if it matters.
return (
            <div className={curtainClasses} style={this.props.curtainClass ? null : curtainStyle} onClick={this.props.clickHandler}>
                <div key="_srm-modal" style={this.props.modalClass ? null : modalStyle} className={modalClasses}>
                    {this.props.children}
                </div>
            </div>
        );
renders (in my app):
<div class="_srm-curtain modal-background" data-reactid=".0.0.1.1.0.0">
    <div class="modal-pod" data-reactid=".0.0.1.1.0.0.$_srm-modal">
        <h1 data-reactid=".0.0.1.1.0.0.$_srm-modal.0">Hello World</h1>
    </div>
</div>
However, if I try to mess with a CSSTransitionGroup:
    let ReactCSSTransitionGroup = React.addons.CSSTransitionGroup;
        return (
            <div className={curtainClasses} style={this.props.curtainClass ? null : curtainStyle} onClick={this.props.clickHandler}>
                <ReactCSSTransitionGroup transitionName={this.props.transitionName}>
                    <div key="_srm-modal" style={this.props.modalClass ? null : modalStyle} className={modalClasses}>
                        {this.props.children}
                    </div>
                </ReactCSSTransitionGroup>
            </div>
        );
renders with these extra spans:
<div class="_srm-curtain modal-background" data-reactid=".0.0.1.1.0.0">
    <span data-reactid=".0.0.1.1.0.0.0">
        <div class="modal-pod" data-reactid=".0.0.1.1.0.0.0.$=1$_srm-modal">
            <h1 data-reactid=".0.0.1.1.0.0.0.$=1$_srm-modal.0">Hello World</h1>
        </div>
    </span>
</div>
If I'm doing something wrong, please point me in the right direction! Maybe there was a note in the docs I missed!
Thanks guys, keep up the great work!
I believe this is the documented behavior of the CSSTransitionGroup.
Thank you so much! I missed that byline! @garetht
This issue might have been closed, but using React.Fragment as component property solved the issue for me.
<ReactCSSTransitionGroup
    component={React.Fragment}
    transitionName="slide"
    transitionEnterTimeout={500}
    transitionLeaveTimeout={500}
 >
 ....
</ReactCSSTransitionGroup>
Most helpful comment
This issue might have been closed, but using React.Fragment as component property solved the issue for me.